To study ontogenetic variation in the mechanisms that control operant responding, infant rats 4-16 days old were trained to lift a forelimb in order to receive an infusion of a sucrose-milk solution (reinforcer). Although the learning processes that contribute to this behavior were to some extent functional in pups 5-6 days old (Experiment V), there were major age-related constraints on the performance of this response. We were unable to observe selective responding with the reinforced forelimb until pups were 9 days old (Experiment I). This was in part because pups tested when 7, but not 9 days old, were highly activated (as measured by responding with both forelimbs) by both the reinforcer (Experiment II) and cues associated with the reinforcer during training (Experiment IV). Implications of these findings are discussed.