Paralyzing customs inspections of Mexican border traffic and other expressions of U.S. dissatisfaction with Mexico's search for a kidnaped U.S. narcotics agent have angered Mexican leaders, prompting the delivery of a diplomatic protest note.

For the past four days, Mexican motorists crossing into the United States have been subjected to exhaustive inspections by U.S. customs officers, said to be seeking clues to the Feb. 7 abduction in Guadalajara of Enrique Camarena Salazar, a U.S. Drug Enforcement Administration official. The resulting massive border tie-ups have been "clearly a way of putting pressure on the Mexicans to shape up and get something going" in the Camarena investigation, one informed U.S. source said.

In response, Foreign Minister Bernardo Sepulveda ordered Mexico's ambassador in Washington, Jorge Espinoza de los Reyes, to present to the State Department a diplomatic note demanding an explanation of the measures, Sepulveda said last night. Breaking with precedent, the United States did not inform the Mexican government of the border action beforehand, Sepulveda said.

A State Department spokeswoman in Washington declined to comment on the report of the diplomatic protest.

In Ciudad Juarez, the sprawling border twin of El Paso, Tex., politicians and business leaders announced an attempt to organize a boycott by Mexicans of U.S. stores to protest what they called "harassment" of Mexican motorists. Mexicans accustomed to quick U.S. shopping trips must now wait three hours or more to pass through El Paso's border posts, according to reports from the border.

"We haven't found any justification whatsoever" for the U.S. action, Sepulveda said in an interview on Mexico's state television network last night.

There have been no breakthroughs in the Camarena case, and U.S. law enforcement officials feel that Mexico "is not pursuing its investigation with vigor and intensity," a U.S. source said. Attorney General William French Smith last week sent a cable to his Mexican counterpart, Sergio Garcia Ramirez, also "expressing frustration and disappointment" with the Mexican response, the U.S. source confirmed.

Mexican officials voiced resentment at this U.S. criticism, saying they have deployed nearly 100 federal police agents and 200 state policemen in the search for Camarena. The unannounced border clampdown is unlikely to have the desired effect of catalyzing an expanded search effort and can only chill relations further, they warned.

One factor slowing the investigation, some U.S. officials suggested, could be the influence of Mexican drug rings over some local law enforcement authorities. Camarena, at the time of his kidnaping, was believed to have been investigating the bribery by narcotics traffickers of unnamed officials, Mexican police spokesmen said.

If U.S. authorities "have evidence of inefficiency in Mexico's police forces, they should present it to us formally, without resorting first to these kinds of pressures," an aide to President Miguel de la Madrid said.

U.S. Ambassador John Gavin met with de la Madrid privately yesterday afternoon to discuss the issue, U.S. and Mexican sources confirmed today. One item believed to have been discussed was the possible issuance by the State Department of an official "travel advisory" warning U.S. tourists of the potential risks of a visit to Guadalajara and environs.

Camarena was the seventh American to be kidnaped in Guadalajara in three months, U.S. diplomats here noted.

On Dec. 2, two American couples who had been proselytizing for the Jehovah's Witnesses in Guadalajara were abducted in a residential neighborhood.

John Walker, a Minnesota Vietnam veteran who had lived in Guadalajara for the past year on a disability pension, disappeared Jan. 30 or 31 along with a visiting friend, Alberto Radelat of Texas.