For veterinarians treating livestock in the field or making house calls to pets, traveling in vehicles outfitted with much of the equipment of a brick-and-mortar hospital is a given. Yet the legality of an essential component of mobile practice has been in question since 2012 when California veterinarians with mobile practices registered to their home addresses began receiving letters from the U.S. Drug Enforcement Agency (DEA) warning that they were in violation of federal law for transporting controlled substances.