The blocking phenomenon was investigated in the sexual response system of male Japanese quail. Access to a live female quail served as the unconditioned stimulus (US). The same audiovisual cue served as the pretrained stimulus in all ofthe experiments. Following asymptotic conditioning ofthe audiovisual cue, a second conditioned stimulus (CS2) was added. In Experiment 1, CS2 was a rectangular wood block that had little or no resemblance to a female quail and could not support copulatory behavior. In Experiment 2, CS2 was a terrycloth object that had no quail parts but could support copulatory behavior, and, in Experiment 3, CS2 was a terrycloth object that had a taxidermically prepared head of a female quail added. The terrycloth-only object supported more rapid conditioning than did the wood block, but the blocking effect was obtained with both kinds of stimuli. Approach responding to the terrycloth + head object required pairing it with copulatory opportunity, and the terrycloth + head object supported at least as rapid conditioning as did the terrycloth-only object. However, responding to the terrycloth + head object was not blocked by the pretrained audiovisual cue. These results indicate that the blocking effect occurs in sexual conditioning even with stimulus objects that can support copulation. However, the addition of species-typical head cues to an object makes that object such a powerful stimulus that conditioned approach responding to it cannot be blocked by a previously conditioned arbitrary audiovisual cue.One of the cornerstones of contemporary investigations and interpretations of Pavlovian conditioning is the blocking effect, which was originally demonstrated by Kamin (1969) in a standard fear conditioning preparation with rat subjects. In the first phase of the blocking experiment, the rats received a noise conditioned stimulus (CS) paired with a shock unconditioned stimulus (US).