Along 16th Street, just a few blocks from the White House, stands a gray, four-story building bristling with electronic devices and loaded, some US officials say, with spies.
The building is the Soviet Embassy. And if figures from the FBI are accurate, about 100 of the Soviets there work either for the KGB or for Soviet military intelligence, the GRU.
Across the United States, perhaps 400 Soviet agents ply their trade - based in Washington, or at the United Nations in New York, or at the consulate in San Francisco.
Now Congress - still angered by the Soviet shoot-down of Korean Air Lines Flight 7 - wants to do something about these alleged spies.
The Senate has voted for a resolution that urges President Reagan to reduce the number of Soviet officials in the US by about 100.
A number of senators, led by Walter D. Huddleston (D) of Kentucky and Jesse A. Helms (R) of North Carolina, are also urging that other steps be taken to put Soviet and US diplomats on a more equal footing.
All this upsets some people at the State Department and the Central Intelligence Agency. They are concerned that if the US gets tough on Soviet spies, Moscow is sure to reciprocate. If they do, the US is going to come out the loser, they argue privately.
Not so, says the Senate. Lawmakers point to the wording of their resolution, which calls for equal treatment - not special advantage - for the US.
The resolution insists that there be ''substantial equivalence'' in the number of employees of both US and Soviet missions. It also insists that Soviet officials here have the same travel and living restrictions as US diplomats in Moscow.
The arguments over this issue go as far back as the 1940s and 1950s.
At the beginning of World War II, US diplomats were the toast of Moscow. The Kremlin desperately needed American help against the Nazi Blitzkrieg. Americans were able to travel in the USSR with relative freedom.
The chill of the cold war changed that. By the early 1950s, the Soviets had enacted strict travel rules for Americans and other foreigners. By the mid-1950s , the US had imposed similar rules on the Soviets here.
Even so, Senator Huddleston and others argue that treatment remains dissimilar. They are particularly troubled by several things:
* Unequal numbers of employees.
Each side has about 200 diplomats accredited to the other's country. However, the actual numbers are larger than that. The US actually has about 320 American employees in the Soviet Union. But the Soviets, including officials with such government agencies as Intourist and Amtorg, have about 425.
* The new embassies will give the Soviets an advantage.
Each side is building a new embassy. However, the Soviet Embassy in Washington is going up in an area known as Mount Alto, near the Washington Cathedral. This is the highest point of ground in the city, and it will give the Soviets an advantage in electronic eavesdropping. The new US Embassy, on the other hand, is on one of the lowest points of ground in Moscow - a plot of ground some critics label a ''swamp.''
* Lack of restrictions on UN employees.
The Soviets, in addition to their regular diplomats and trade officials, have 520 nationals working at the United Nations. The FBI estimates that about 40 percent of these, or more than 200, work for Soviet intelligence.
Strict travel requirements cannot be applied to Soviet UN employees, who fall under UN rules and regulations. As a result, it is far easier for Soviet workers at the UN to crisscross the US, and collect information.
* Electronic eavesdropping, especially in San Francisco. When the US opened a consulate in Leningrad (mainly to help American tourists), the Soviets were permitted to open a consulate in San Francisco.
The Soviets, one Senate specialist says, have 41 employees in San Francisco who are electronically tapping a ''gold mine'' of information from nearby Silicon Valley, the heart of America's high-technology industry.
The Senate resolution soon goes to a House-Senate conference, where administration officials hope it will be stopped. The State Department, in particular, worries about the Senate's get-tough mood for several reasons.
''We toss out 100, and maybe Moscow will do the same, and before you know it, our embassy will no longer be effective,'' says one analyst. ''And who will that hurt more - the Soviets or us?''
Says another official:
''The Soviets can just read Aviation Week or the Washington Post or The Christian Science Monitor and learn a tremendous amount about the United States. But the Soviet Union is a closed society. We need every window on the Soviets that we can get, and this will just lead to closing some of those windows.''
If anything, argues Sen. Daniel Patrick Moynihan (D) of New York, one senator who has been cautious about the Huddleston resolution, the US needs more diplomats in the USSR, not fewer. The US should be trying to expand its presence in the Soviet Union, not doing things that could reduce it.
It is also important to note, several sources say, that the US has 225 to 250 ''local hires'' who work at the US Embassy in Moscow. These are Soviet, Italian, Mexican, and other employees who act as chauffeurs, janitors, cooks, dishwashers , telephone operators, etc. The Soviets won't hire local people in Washington - they are afraid. So their extra numbers are overstated, it is argued.
There is a difference in living conditions, all sides agree. But, says one analyst: ''The Soviets give us the best they have. Compared with the average Soviet citizen, the living conditions of our diplomats there are palatial.''