Zedillo's Challenge

LAST year's assassinations of two prominent Mexican politicians helped plunge the country into its present political and economic turmoil. The investigations of those killings, unsettling in the short term, could eventually help Mexico climb toward stable democracy.

This week's arrest of a brother of former president Carlos Salinas de Gortari as a suspect in the murder last September of Jose Francisco Ruiz Massieu, a well-known reformist political figure, breaks new ground in more ways than one. It marks the end of virtual immunity for the families of top leaders in the long-ruling Institutional Revolutionary Party (PRI). And it could mark a breakthrough in the Massieu case, though the evidence made public so far about Raul Salinas's involvement is inconclusive.

The apparent vigor of the probe into the Massieu killing and the earlier slaying of PRI presidential candidate Luis Donaldo Colosio could give needed thrust to the fledgling presidency of Ernesto Zedillo Ponce de Leon. A collapsing peso and half-hearted efforts to stop the rebellion in Chiapas have cast doubts on Mr. Zedillo's mettle. The Salinas arrest, if substantiated, and the uncovering of a conspiracy in the Colosio assassination could ease some of those doubts.

In the best light, events in Mexico can be seen as evidence of movement toward equal justice and the rule of law -- themes emphasized by President Zedillo. That should mean, over the long haul, a restoration of confidence among both Mexicans and foreign investors.

A darker side to these events is what they reveal about rifts within Mexican society and the ruling party itself.

The PRI's glue of mutual interests and personal connections, which has held since 1929, is coming unstuck, and people who profited under the old system feel threatened.

Zedillo, whose background is more on the fringe of the PRI ''family'' than in its bosom, may be well positioned to pursue greater openness and democracy. But he has hesitated to use the concentrated powers of the presidency to impose change -- for instance, by settling disputed state governorship elections. President Salinas had no such qualms.

The biggest challenge ahead for the Mexican leader may be figuring out how to use some of the powers of the old regime in order to help forge a new one.