It was fun while it lasted but there's no doubt now that for California's agricultural exporters the honeymoon with Mexico is over.

Across the board, be it wine, tomatoes, peaches, pears or plums, there's general pessimism about the future of exporting south of the border - regardless of the promise of the Nafta. And these views aren't just based on the sorry state of the Mexican economy.The complaints range from the persistence of border-crossing headaches to suggestions of a near-conspiracy among some Mexican importers.

"This is a bad dream come true," said Ed Beckman, president of the California Tomato Board, based in Fresno, Calif.

For tomato exporters, in fact, it's more of a nightmare. At first, the Nafta gave the state's growers a big shot in the arm. In 1992, it exported only 600,000 25-pound cartons to Mexico. That figure jumped to 1.3 million cartons in 1993 and a whopping 2.25 million in 1994. Then, with the peso devaluation crisis, among the country's other economic problems, the party ended. So far this year, exports to Mexico are down 95 percent.

A similar story is told by the state's wine industry. According to Joe Rollo, director of the international department of the Wine Institute, which is based in San Francisco, exports to Mexico went from $2.3 million in 1990 to an all-time high of $6.8 million last year. In the first five months of 1995, shipments have reached only $547,000. In the same period last year they topped $2.8 million. (1994 worldwide exports reached $191.8 million.)

Over at the Western Growers Association, which represents some 2,500 growers and shippers of fresh fruits and vegetables in California and Arizona, last year's optimism also has waned.

''Our exports will be down this year," predicts Matt McInerney, senior vice president.

Some stone fruit exporters have gone beyond pessimism to outright anger.

"I got a fax in here from one grower who swore that he would never, ever again export to Mexico," an official of the state's agricultural department said.

It was such a promising beginning. According to state government statistics, in 1994 agricultural trade with Mexico increased an unprecedented 50 percent, or by more than $345 million over 1993. Raisins, almonds, and low- fat milk experienced double- and triple-digit growth.

Overall, 1994 was a good year for the industry. Agricultural exports rose by 10 percent last year to $11.8 billion. Mexico became the No. 3 destination for California fresh fruits and vegetables.

So what happened? To be sure, the devaluation of the peso dramatically decreased consumer appetites for imported foodstuffs. And importers who before were able to get credit found themselves cut off - first by their own banks, many of which failed, and then by U.S. institutions.

Mike Binn, sales manager for Blue Anchor Inc., of Dinuba, Calif., a co-op that ships for growers of various tree fruits, said he now insists on payment ''up front" from Mexico in U.S. dollars or financing from the exporter.

But border crossing problems - particularly, simple paperwork issues - also never seemed to get resolved. The Tomato Board's Mr. Beckman reports that Mexican officials routinely request additional documents before letting a shipment pass.