When rats are paired in a situation where the reward is contingent on a bar-pressing response, one of the rats performs most of the bar pressing (worker rat), while its partner receives the reward without working (parasite rat). The possible influence of the persistence of a learned response and of the schedule of reinforcement, was studied. No correlation was found between persistence of response and the categorization of the rats as workers or parasites. On the other hand, when rats individually trained in a variable interval (VI) schedule of reinforcement were paired with rats trained under continuous reinforcement (CRF), most, although not all, VI trained animals became workers. This result shows that rats can be induced to behave as workers through the manipulation of the schedule of reinforcement at the individual training.