1. In the rat, projections from the suprachiasmatic nucleus (SCN) to the supraoptic nucleus (SON) of the hypothalamus were characterized in vivo using extracellular recordings and in slice preparations using both extracellular and whole-cell patch clamp recording. 2. Of 117 magnocellular neurones recorded in the SON in vivo, fifteen (13%) displayed a short latency excitation, sixty-eight (58%) a short latency inhibition, six (5%) were unresponsive and twenty-eight (24%) gave long latency responses following SCN stimulation. 3. The responses of putative vasopressin cells in the SON to SCN stimulation in vivo (4 out of 61 cells, 7% excited; 49 out of 61 cells, 80% inhibited) were significantly different from those of putative oxytocin cells (10 out of 50 cells, 20% excited and 16 out of 50 cells, 32% inhibited; P < 0.02, test for differences between proportions). 4. Recordings in vitro using patch technology in whole-cell mode showed both inward and outward currents in SON cells at holding potentials near resting membrane potential following stimulation of the SCN region. The outward currents could be blocked by bicuculline (10 microM; n = 7) and the inward currents were blocked by the non-NMDA antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (5 microM; n = 4). 5. We conclude that there is a strong projection from the SCN to the SON with both inhibitory (GABAergic) and excitatory (glutamatergic) components which may regulate the daily changes in neurohypophysial hormone secretion.