Two men were arrested Wednesday morning following a 12:10 a.m. traffic stop on Highway 29 north of Highway 12, according to the California Highway Patrol.
The 1996 GMC minivan with three men in it was stopped because of a registration violation, the CHP said.
A loaded gun that had been reported stolen in Texas was found in a vehicle compartment, CHP Officer Steven Dundas said.
Darwin Calvario, 23, of Napa, was arrested and booked into the Napa County jail on suspicion of carrying a concealed weapon, according to the jail logs.
Emilio Cervantes, 28, of Napa, was arrested and booked into the Napa County jail on suspicion of driving under the influence, Dundas said. The third occupant was not arrested.