Retzius (Rz) neurons in the midbody ganglia of medicinal leeches responded to ACh, applied to their somata, in a manner that depended upon the neuron's segmental location: Rz neurons in ganglia from midbody segments 5 and 6 [Rz(5,6)] hyperpolarized, whereas Rz neurons from all other segments [Rz(X)] depolarized. Midbody segments 5 and 6 are notable because they contain the male and female reproductive organs. Both types of Rz neurons responded to ACh in a complex way, but the initial phase of each response appeared to be nicotinic because nicotinic agonists evoked the responses and nicotinic antagonists blocked them. The reversal potentials of the responses and the effects of changing the internal and external Cl- concentration indicated that the hyperpolarizing response of Rz(5,6) neurons depended upon Cl- whereas the depolarizing response of Rz(X) neurons did not. The segmentally characteristic responses of Rz neurons arose during embryonic development. Removing the reproductive ducts [the peripheral targets of Rz(5,6)] early in embryogenesis caused the Rz(5,6) neurons to depolarize in response to ACh rather than to hyperpolarize. This result indicates that development of the characteristic response of Rz neurons to ACh is strongly influenced by interactions between the neurons and their appropriate target tissues.