Abstract It has previously been porposed that Random-Interval schedules fail to induce polydipsic drinking in rats because such schedules fail to provide gradients of reinforcement probability. In the present study, food-deprived rats received single pellet meals delivered on either a Fixed-Time or a Random-Time 120-sec schedule, with half of each group receiving signalled, and half unsignalled, pellet deliveries. Acquisition of polydipsia was retarded in animals exposed to the Random-Time Unsignalled condition. However, the imposition of a CS+ for pellet delivery on the identical Random-Time schedule resulted in levels of polydipsia comparable to those obtained in both Fixed-Time conditions. These results suggest that the establishment of a Pavlovian conditioned inhibitory signal, as well as local periods of low reinforcement probability, may be sufficient for the development of schedule-induced behaviors such as polydipsia.