In order to determine the responses of paraventricular nucleus magnocellular neurones following activation of central serotonergic pathways, single unit activity was recorded and responses following electrical stimulation of the midbrain dorsal raphe nucleus were examined. Approximately one third (32%) of the phasically active, vasopressin-secreting neurones were inhibited by the stimulation, the remaining such cells being nonresponsive. In contrast, only two of the non-phasic cells (13%) were inhibited by the stimulation whilst 53% were excited (p less than 0.005, chi2-test). The onset latency of both inhibitory and excitatory responses were similar, whilst offset of the inhibitory responses was about twice that of the excitatory responses (p less than 0.005, t-test). Two of the non-phasic cells were antidromically identified as projecting to the dorsal raphe. The results obtained indicate a role for dorsal raphe projections to the paraventricular nucleus in the regulation of neurohypophysial hormone secretion. The observation that different sub-populations of the cells recorded showed different responses, suggests that several mechanisms may be involved in the control of neuronal activity in the region recorded, in response to activation of the central serotonergic pathway examined. The results obtained are intended to further clarify the neural mechanisms regulating the secretion of vasopressin and oxytocin from the neurohypophysis.