Mexico is bowing to market pressure and cutting its crude oil price by about a dollar per barrel, the government announced tonight.

Affecting both the heavy Maya and light Istmus oil marketed by Mexico, the price reduction is seen as Mexico's most significant break with OPEC's oil-marketing guidelines since the present Mexican government took power 2 1/2 years ago. Mexico is not a member of the cartel but has adhered closely to OPEC's price and production accords.

Following the most recent inconclusive OPEC meeting in Vienna, Mexico decided that it "couldn't tolerate a situation in which it was being progressively forced out of its traditional markets by the irregular marketing practices of other exporters," Petroleos Mexicanos and the Ministry of Energy and State Industry said in a joint statement issued late tonight.

U.S. oil companies, which purchase half of Mexico's exports, will now pay $26.75 for Mexico's light oil and $23.50 for its heavy crude, down from the last posted prices of $27.75 and $24 respectively.

In Asia, Isthmus will be sold for $26.50 and Maya for $23 per barrel. Similarly, Mexico's prices for European clients will drop to $26.25 for the light oil and $22.50 for the heavy crude.

The OPEC benchmark price is $28 a barrel, but several members have been selling their oil at discounts.