Partial results show Mexico's main opposition party has won victories in two of the four states that held gubernatorial elections on Sunday.
Incomplete tallies and exit polls show the Institutional Revolutionary Party (PRI) defeating the ruling National Action Party (PAN) in the states of Puebla and Tamaulipas.
The PRI is also leading in Sinaloa, while early results in the state of Tlaxcala show a tight race between the PRI and PAN candidates.
The PRI currently holds the governorship in all four states.
Gubernatorial races are seen as crucial, as political parties maneuver ahead of Mexico's 2006 presidential elections.
The PRI ruled Mexico for 71 years until it lost the 2000 elections to President Vicente Fox's National Action Party.
Some information for this report provided by AP.