The responding maintained by a reinforcer depends on the relation of the reinforcer not merely to the response that produces it but also to other preceding responses. Early responses in a sequence that ends in a reinforcing consequence make smaller contributions to later response rates than more recent ones, by virtue of the longer delays that separate them from the reinforcer. This study shows that the relation between a response and a later reinforcer contributes to responding only if no other reinforcers intervene; in other words, each reinforcer blocks responses that precede it from the effects of later reinforcers. Pigeons' pecks were maintained by fixed‐interval (FI) schedules of food reinforcement. When FI 60‐s (short) and FI 75‐s (long) schedules began simultaneously within constant 150‐s cycles, long FIs did not affect short‐FI performances, but short FIs eliminated the first 60 s of long‐FI performances. Removing either short‐FI reinforcers or short‐FI stimuli showed that short‐FI reinforcers and not short‐FI stimuli blocked the first 60 s of the long‐FI performance from the retroactive effects of the long‐FI reinforcer. With FI 15‐s and FI 75‐s schedules, the short‐FI reinforcer was followed by reduced long‐FI responding, but a schedule that prevented discrimination based on time since a reinforcer eliminated this proactive effect of the short‐FI reinforcer. In other words, the retroactive effects were reinforcer effects whereas the proactive effects were discriminative effects. Quantitative descriptions of variable‐interval performances, in which reinforcer effects may operate in the absence of temporal discriminative effects,