A federal Drug Enforcement Administration agent said today that drug traffickers suspected of kidnaping a fellow DEA agent Feb. 7 in Guadalajara, Mexico, are believed to have joint drug operations with Colombian traffickers.

Arthur Sedillo, a DEA agent stationed in Mexico, identified the suspects as members of the Miguel Felix Garrardo and Rafael Caro Quiterro drug-trafficking families. "They have merged their activities, and they are not restricted to one drug," Sedillo said in testimony before the President's Commission on Organized Crime, which completed two days of hearings on heroin trafficking here today. "They are planting opium and marijuana . . . . "

The abduction of veteran DEA agent Enrique Camarena Salazar has come up several times during the hearings.

Deputy DEA Administrator John C. Lawn testified Wednesday that drug traffickers in the Guadalajara area have threatened eye witnesses to the abduction.

"They have been advised that they should not cooperate in the investigation," Lawn said. He added, "There was a reluctance on the part of law enforcement authorities in Guadalajara and Mexico City to initiate an investigation."

In Washington, Mexican diplomats said Mexican Ambassador Jorge Espinosa de los Reyes met Deputy Secretary of State Kenneth Dam and expressed "deep concern" over car-by-car searches at the U.S.-Mexican border for leads in the Camarena kidnaping, United Press International reported.

At the same time, U.S. Ambassador to Mexico John Gavin, who returned to Washington for consultations Wednesday night, held a round of meetings at the State Department on strained relations between the two neighbors.

Officials said a travel advisory urging Americans to show caution in traveling to Mexico is under consideration in response to a recent rash of kidnapings of Americans.

The Congressional Border Caucus, meanwhile, held an emergency session and voted to send a letter to President Reagan, demanding an end to the inspections.

Later, State Department press officer Brian Carlson said in recent months, seven U.S. citizens "have disappeared and are thought to have been kidnaped" in the Guadalajara area. He identified them as Rose and Dennis Carlson, Patricia and Benjamin Mascarenas, Camarena, John Clay Walker and Alberto Radelat, the Associated Press reported.