Electrophysiological and pharmacological studies have been performed on a clone of mouse hypothalamic neurosecretory cells synthesizing neurophysin and vasopressin (HT9-C7). These neurons possessed low resting membrane potential (RMP) and weak membrane resistance (MR). They did not exhibit spontaneous activity. Electrical stimulation or microiontophoretic application of putative neurotransmitters did not induce action potentials. Nevertheless, dopamine and gamma-aminobutyric acid appeared to exert a slight hyperpolarizing effect on RMP. Radioimmunoassays, carried out on the culture medium after electrical stimulation, did not reveal any measurable quantities of vasopressin. However, an electron microscopic analysis of the cytoplasmic processes of these cells did not reveal axonal outgrowth. It can be assumed that the weak electrophysiological and pharmacological properties of these neurons have to be related to their weak morphological differentiation. Two hypotheses might account for the absence of most characteristics of in situ magnocellular hypothalamic neurons in HT9-C7 cells: the lack of pituicytes, the target cells for the axon terminal of magnocellular neurons, and the SV40 transformation itself which may impede neuronal maturation.