Four experiments compared the extinction of responding to a continuously reinforced (CRf) conditioned stimulus (conditional stimulus [CS]) consistently reinforced on every trial, with extinction of responding to a partially reinforced (PRf) CS that had been inconsistently reinforced. To equate the acquisition of responding between the two CSs, the average duration of the CRf CS was extended so that it scheduled the same overall rate of reinforcement per unit time as the PRf CS. Experiment 1 used a within-subjects design to compare the rates of extinction for a 10-s PRf CS reinforced on 33% of trials versus a 30-s CRf CS. Experiment 2 made the same comparison but using a between-subjects design. Experiment 3 compared extinction in a group trained with a 10-s PRf CS reinforced on 20% of trials and a group trained with a 50-s CRf CS. Experiment 4 compared the rates of extinction following two partial reinforcement schedules: a 10-s PRf CS reinforced on 33% of trial versus a 20-s CRf CS reinforced on 66% of trials. In each experiment, responding took longer to extinguish for the CS that scheduled a lower per-trial probability of reinforcement. Modeling of individual extinction curves using Weibull functions indicated that the latency to initiate extinction was directly related to the per-trial probability of reinforcement learned during acquisition. For example, compared with training with a CRf CS, rats reinforced on 33% of trials took approximately 3 times as many trials to initiate extinction, and rats reinforced on 20% of trials took 5 times as many trials to initiate extinction. These results provide support for trial-based accounts of extinction (e.g., Capaldi & Deutsch, 1967), whereby rats learn about the expected number of trials per reinforcer, and extinction depends on the number of expected reinforcers that have been omitted rather than on the number of extinction trials per se. (PsycINFO Database Record (c) 2019 APA, all rights reserved).