The larval-pupal transformation of the tobacco hornworm, Manduca sexta, involves the loss of many larval-specific behaviors, including a simple withdrawal reflex of the abdominal prolegs. This reflex is mediated by monosynaptic excitatory connections between afferents that innervate mechanosensory hairs on the proleg [the planta hairs (PHs)] and motoneurons that innervate proleg retractor muscles. In response to hormonal cues during the final days of larval life, the dendritic arbors of proleg retractor motoneurons regress extensively. Intracellular recordings indicate that the synaptic depolarization evoked in proleg motoneurons by electrical stimulation of PH afferents decreases by 78% during the larval-pupal transformation, over the same time course that the motoneuron dendrites regress. During the same period the number of PH afferents and the extent of their central projections may decrease slightly. To test the relative contributions of developmental changes in the presynaptic afferents and postsynaptic motoneurons to the weakening of the reflex pathway, we generated heterochronic mosaic pupae that retained one larval proleg. The PH afferents on the retained proleg retained their larval characteristics while their postsynaptic targets, the proleg motoneurons, appeared to regress normally. In the mosaic hemisegments, electrical stimulation of PH afferents evoked only a small synaptic depolarization of the proleg motoneurons, similar in amplitude to that recorded in normally developing pupae; thus, the developmental status of the afferents was irrelevant to the loss of the reflex. These findings suggest that postsynaptic changes, e.g., structural regression of motoneuron dendrites, may cause the weakening of the reflex pathway. This is the first demonstrated correlation between the hormonally mediated regression of motoneuron dendrites and the loss of a specific behavior during metamorphosis.