AbstractExtracellular single unit activity was recorded from 73 neurones in the median preoptic nucleus (MnPO), identified by antidromic activation as projecting to the supraoptic nucleus (SON) area in urethane‐anaesthetized male rats. Thirteen of 73 identified MnPO neurones were silent, and 44 of 60 spontaneously active MnPO neurones were tested for their responses to electrical stimulation of the nucleus tractus solitarius (NTS). The cells were divided into 4 groups according to their responses; those which were excited orthodromically (OD+; n = 15), those which were unresponsive (UN; n = 21), those which were inhibited orthodromically (OD−; n=4), those which showed initial inhibition followed by excitation (OD+n = 4). Some of these neurones were further tested for their responses to haemorrhage and/or increase in blood pressure produced by intravenous administration of the oe‐agonist, phenylephrine, and/or to hyperosmotic stimulation produced by intraperitoneal injection of 1.5 M NaCl. Six out of 10 OD+cells were excited by haemorrhage, 6 out of 11 OD+cells were inhibited by phenylephrine, and 5 out of 9 OD+cells were excited by hypertonic saline. On the other hand the UN cells tended to be unresponsive to each type of stimulus. Three out of 7 OD+cells were excited by both haemorrhage and hypertonic saline, and 3 out of 8 OD+cells were inhibited by phenylephrine and excited by hypertonic saline. The results may suggest that MnPO neurones which receive afferent input from the NTS may be sensitive not only to haemodynamic change but also to change in plasma osmotic pressure and that such population of MnPO neurones may integrate a part of the haemodynamic and osmotic information and contribute to the control of neurohypophysial hormone re