Cracks are beginning to appear in the armed services' all-volunteer system which could have military significance and political impact. * Army recruiters are finding that fewer top-quality young men are beating on their doors. During the first quarter of fiscal year 1984, that figure dropped 22 percent over the previous year.
* Vice-Adm. William P. Lawrence, chief of naval personnel, says ''the recruiting and retention environment ... appears to be changing in ways which will make it increasingly more difficult to continue meeting our manpower requirements.''
* Lt. Gen. Kenneth L. Peek Jr., Vice-Admiral Lawrence's Air Force counterpart , warns of a possible ''hemorrhage of talent'' as the economy improves just as the services need even more men and women of increased talents to maintain and operate the sophisticated new equipment now filling the nation's arsenal.
* Private studies for the Army say a return to the draft may be necessary. New reports by the Brookings Institution and the Heritage Foundation raise the possibility as well.
For the moment, the armed services are handily meeting their recruiting goals , and the quality of new recruits - as measured by the number with high-school diplomas - is better than ever. Reenlistment rates also are setting records, according to the Pentagon. But military officers and civilian manpower experts warn that this could well change.
There are two fundamental causes: an improving economy, which makes military service less attractive; and a declining ''youth cohort'' (healthy 19-year-old males) due to the tailing off of the baby boom. This group will drop by nearly one-fourth by 1993.
When college-bound youths are subtracted (and financially pressed colleges are actively recruiting as well), this means that more than half of the rest would have to volunteer to meet future service requirements.
''There are good reasons to ask whether the successes of the early 1980s can be sustained, given the prospect that the military could run up against an adverse supply-and-demand situation later in the decade,'' writes Martin Binkin in the Brookings study.
Mr. Binkin says it is ''doubtful that adjusting military pay or tinkering with manpower policies could prevent serious recruitment shortfalls.'' And he warns that plans for force expansion and weapons modernization could leave the US ''fielding military forces in the 1990s whose effectiveness would depend on a military draft, only to find a citizenry unwilling to support it.''
Lt. Col. Robert K. Griffith, a US Army historian, warns that many men in reserve units (which the Defense Department counts on to fill out active-duty forces during wartime) may not respond when called to fight.
''There are likely to be greater shortages than the Army is willing to concede,'' he writes in a recent study for the Heritage Foundation. ''Under assumptions concerning a war in Europe today, the US forces would be short of trained manpower by as much as 179,000 troops before the first volunteers and newly trained draftess began to arrive.''
The manpower issue is at the core of the recent controversy over readiness and combat capability. Personnel is a key factor here, especially the quality of recruits and ability to keep them in service.
Will there be a return to military conscription anytime soon? This is highly unlikely, despite July's US Supreme Court decision allowing the federal government to tie draft registration with student loans.
The only high-level public official in favor of a new military draft is Sen. Ernest Hollings (D) of South Carolina, whose bid for the Democratic presidential nomination sputtered ealier this year.
President Reagan had to break a 1980 campaign pledge just to approve the current rather mild registration program. The armed services themselves - reflecting their post-Vietnam nervousness about public support - are hesitant to push for anything as politically unpopular as a military draft, especially when they are winning bigger budgets.
The administration wants to alleviate any potential military manpower problems with pay raises for those in service, more education benefits (although not a blanket GI Bill as many lawmakers are urging), and increased resources for recruiting.
In addition to pay and benefits improvements, argues Lieutenant Colonel Griffith, the President should have the authority ''to induct 100,000 draftees for six to 12 months training and service without prior consent from Congress.'' And, he says, those now required to register should also be tested and classified for potential service.