In vitro intracellular recordings of membrane potential obtained from the oxytocin and vasopressin neurons of the mammalian hypothalamo-neurohypophysial system in slices (1-3) and expiants (4, 5) have demonstrated many of the intrinsic properties of these magnocellular neuroendocrine cells (MNCs). Voltage-clamp techniques, which are required to study directly the currents underlying intrinsic or transmitter-evoked potential changes, have been applied to cultured embryonic (6) or neonatal supraoptic neurons (7-9) and have been successfully applied to adult supraoptic neurons in situ in only one laboratory (10, 11). We have modified a technique for dissociation of viable adult guineapig hippocampal neurons (12) to dissociate supraoptic MNCs from adult rats for voltage-clamp studies. MNCs were selectively labelled with a fluorescent dye in vivo so that they could be identified after dissociation and prior to making recordings. These data have been published in abstract form elsewhere (13, 14).