MEXICO'S ejido system of communal land ownership was formed in response to a long history of exploiting the poor. During the Revolution of 1910-17, peasants demanded their right to the land. Large holdings were broken up, put under government control, and parceled out to the poor.This was a gain in social equity. But over the years the inefficiencies in the system became obvious. Mexico's current reform-minded president, Carlos Salinas de Gortari, moved in November to overhaul the ejido system by giving small farmers a new set of rights: outright ownership of their land, with the options to buy more, to sell, or to rent. Mr. Salinas's action brought quick alarms from peasant organizations and opposition parties who worry that the privatization of land ownership could lead to renewed exploitation of the poor. What's to stop investors or developers, who could care less about social equity, from cheating the campesinos out of their land, they ask. Exploitation, however, is relative. The ejidos have not given rural Mexicans the resources needed to rise above poverty. And while poor farmers were freed from the old feudal relationships, the government became a new kind of master, dictating the availability of such basics as fertilizer and credit. Without title to their land, farmers had no collateral to obtain loans in order to upgrade their operations. Under the Salinas plan, that should change. Those who want to expand will be able to buy more land. Foreign investment will play an important role, as it has already in a few instances. Will some farmers be taken advantage of? Probably. Land dealings will need to be carefully watched to avoid a souring of the reform. This reform, like others put forward by Salinas, involves a calculated political risk. Much of the strength of his Institutional Revolutionary Party resides in the rural areas where the ejidos are well established. The communes have always turned out the vote for the ruling party. But Salinas hopes the unleashing of economic opportunities will win a new kind of loyalty from rural voters. A broader goal is to lessen Mexico's divide between rich and poor. Much will depend on how the reforms are implemented - with safeguards against corruption and an eye to educating small farmers about their new rights. A more vital Mexican agriculture has long-range implications both for the North American Free Trade Area now struggling to take shape and for northward emigration. The ejido reform is another step in Mexico's bid to enter the world's political and economic mainstream.