Environment Secretary Alberto Cardenas resigned to seek the presidential nomination of President Vicente Fox's National Action Party.
Cardenas, a 47-year-old industrial engineer, was governor of the western state of Jalisco from 1995 until 2001, and became Fox's top environmental official in September 2003.
He follows former Interior Secretary Santiago Creel into the race for the PAN nomination for the July 2006 election.
Other contenders include former Chihuahua Gov. Francisco Barrio and ex-legislator and party chief Felipe Calderon.