The animals went missing from their stables Saturday night, ahead of the Toro de la Vega festival in the town of Tordesillas, The Local.es reported.
As part of the festival, hundreds of people take on one of the bulls with spears.
If the animal makes it past the fighting zone limits and is standing, the bull is pardoned.
Organizers of the fighting tradition said animal rights protesters may have freed the bulls.
Before the bulls went missing, thousands of people in Madrid protested the festival, The Local.es reported.