1. Extracellular recordings in pentobarbitone anaesthetized male Long-Evans rats examined the influence of electrical stimulation in the diagonal band of Broca on the excitability of 113 putative vasopressin-secreting and 22 putative oxytocin-secreting neurosecretory neurones in the hypothalamic supraoptic nucleus. 2. Single pulse or repetitive (5-20 Hz) stimulation in the ventral part of the diagonal band evoked a prominent reduction in the excitability of 83% of vasopressin-secreting neurones with no effect on the remainder. Amongst oxytocin-secreting neurones, 59% were unresponsive, 27% responded with an increase in activity while only 14% revealed an inhibitory pattern similar to vasopressin-secreting neurones. 3. Diagonal band stimulation-evoked inhibitions were reversibly abolished by local pressure applications of bicuculline methiodide (100 microM) to twenty out of twenty vasopressin secreting cells tested, whereas strychnine sulphate (100 microM) was without effect on four out of four cells tested. 4. In five out of five vasopressin-secreting cells tested, bicuculline applications reversibly abolished the reduction in their activity that follows peripheral baro-receptor activation. Failure to alter baroreflex-evoked depressions in firing during similar trials with prazosin hydrochloride (10 microM, six cells tested), timolol maleate (20 microM, six cells tested) or strychnine sulphate (100 microM, three cells tested) indicated the specificity of bicuculline's action. 5. These findings suggest that a GABAergic pathway from the diagonal band of Broca preferentially innervates vasopressin-secreting neurosecretory supraoptic nucleus (s.o.n.) neurones, and support the view that the baroreflex-induced depression in firing of s.o.n. vasopressin-secreting neurones is mediated in large part through this input.