The generally different form of the functional relations obtained in operant and in classical situations, when these temporal factors are varied, increases our susporal factors are varied, increases our suspicions that are quite different problems. In operant situations, the effect of delay of reinforcement is always to slow acquisition; in classical conditioning situations, there is some temporal interval between the onsets of the conditioned and unconditioned stimuli which is optimal in each situation, and the most rapid acquisition is never obtained if there is no delay between the onsets of these two stimuli. Thus, it is clear that delay of reinforcement is a problem peculiar to operant situations.