In an attempt to clarify the mechanisms involved during synchronous discharge of magnocellular neurosecretory cells of the supraoptic and paraventricular nuclei, extracellular action potentials were recorded from 149 single units located in the rat paraventricular nucleus. Responses of the cells to stimulation of the ipsilateral supraoptic nucleus were recorded and it was observed that 7% of paraventricular nucleus neurones could be antidromically identified as projecting to the supraoptic nucleus. Excitatory responses were recorded from 40% of cells tested and were probably mediated by an interneurone population. Inhibitory responses were recorded from 37% of the cells and may have been mediated by direct projections from the supraoptic nucleus. No differences in responses were recorded from cells identified as projecting to the median eminence or neurohypophysis as compared with the rest of the population tested. However, more of the phasically firing putative vasopressin-secreting cells, (58%) were excited than were continuously active cells (38%). Delivery of short trains of high-frequency stimulation induced bursts of discharge from 93% of cells so tested, regardless of response to single shock stimulation. These burst discharges showed a late onset as compared with responses following single shock stimulation. The results suggest that a complex of intra- and internuclear interactions serve to regulate the neurosecretory activity of magnocellular cells within the supraoptic and paraventricular nuclei.