It has recently been proposed that the probability of an event (e.g. delivery of a reinforcer) can be interpreted as functionally equivalent to the delay of an event. The present experiment examined the equivalence rule that states that reinforcer probability is functionally equivalent to a set of variable delays. A titration procedure was used in which pigeons chose between a standard, probabilistic alternative, and an adjusting, delayed alternative. Reinforcer probability was manipulated by varying the number of keypecks required to obtain a reinforcer. The duration of the adjusting alternative varied as a function of a pigeon's previous choices. There were six standard probabilities. The results indicated that the pigeons were sensitive to changes in the standard probabilities and that there were significant relations between: (1) reinforcer probability and the mean adjusted delay of reinforcement, and (2) the obtained and predicted adjusted delays. The present results support Mazur's conception of the relation between reinforcer probability and delay, and extend his conception to a different method of manipulating reinforcer probability.