1. The activity of single cells in the supraoptic nucleus and the lateral hypothalamus of urethane-anaesthetized rats was studied during electrical stimulation of the neural stalk.2. Neurones activated antidromically from the neural stalk were either unaffected or, rarely, slightly affected by single shocks or by brief trains of shocks presented at an intensity just below that necessary for antidromic invasion. Thus there does not appear to be a strong synaptic coupling between rat supraoptic neurones.3. In contrast, many lateral hypothalamic neurones, and particularly those in the perinuclear zone, were strongly affected by neural stalk stimulation, being either orthodromically excited or inhibited. Thus, rat magnocellular neurosecretory neurones appear to project, directly or indirectly, to the lateral hypothalamus.4. Thirty-one lateral hypothalamic neurones were studied following an I.P. injection of 1 ml. 1.5 M-NaCl. Eighteen neurones responded to this osmotic stimulus with a change of at least 1 spike/sec in mean firing rate.5. Fifteen of the thirty-one neurones were strongly affected by neural stalk stimulation (eight inhibited, seven excited). Ten of these fifteen neurones were affected in the same way by osmotic stimulation. The remaining sixteen out of thirty-one neurones were unresponsive to neural stalk stimulation, and of these, ten were also unresponsive to osmotic stimulation. Thus most lateral hypothalamic neurones responded in a similar way to neural stalk stimulation and to systemic osmotic stimulation.6. Thus the osmosensitivity of some neurones in the lateral hypothalamus may be mediated by synaptic input arising from the magnocellular neurosecretory neurones. However, the osmosensitivity of some lateral hypothalamic neurones cannot be explained in this way, and probably derives from osmoreceptors other than the magnocellular neurones.