Next time you pop across the Mexican border for a visit, remember to leave your assault rifle at home.

This year, 123 U.S. citizens have been arrested in Mexico on weapons charges, according to the U.S. Embassy here, and about 70 Americans are now being held, accused or convicted of violating the country's strict Firearms and Explosives Act.In some cases, people honestly forget they have a gun in the trunk or bullets in the glove compartment, U.S. and Mexican authorities acknowledge. But other cases are more sinister: Mexico is awash in guns smuggled in from the United States and used by organized-crime syndicates, many of them linked to brutal drug cartels.

The Mexican Congress is close to giving final approval to a new law that would give border officials more discretion in cases in which visitors obviously have inadvertently brought weapons with them. But be warned: The law also will make the penalties even harsher for those who do try to smuggle arms into Mexico - already, those convicted face up to 30 years in prison.

More than 1,000 illegal weapons a month were seized from 1995 to mid-1997, nearly 40 percent of them linked to drug trafficking cartels, according to the Foreign Relations Ministry.

Pointing to the flow from the north, Mexican officials like to note, for example, that the gun used to assassinate presidential candidate Luis Donaldo Colosio in 1994 was traced to Texas.

"Just as the Americans pressure us on certain issues, we are going to do the same thing and make this fair," Jesus Silva Herzog, a former ambassador to the United States, told a radio interviewer last year.

His comment came just days after customs officials in San Diego seized two truckloads of illegal weapons, including grenade launchers and automatic rifles, that were about to be smuggled into Mexico.