Spontaneous single-action potentials (units) were recorded extracellularly from explants of the rat organum vasculosum laminae terminalis in vitro. Increasing the osmotic pressure of the bathing solution by 15% by adding NaCl or mannitol increased frequency, whereas reducing the osmotic pressure by 15% by omitting NaCl reduced frequency. The mean frequency ratio (test/control) for 6 (of 13) units responding to a 15% increase was 2.2 +/- 0.5 (SE), and for 8 (of 11) units responding to a 15% decrease it was 0.6 +/- 0.1. These responding units all lay within 55 microns of the ventricular surface. Reduction of the calcium concentration from 2 to 0.75 mmol X l-1 increased the mean frequency of units by 55%. Putative transmitters were added to the bathing solution at 0.1 mol X l-1. The results were as follows: carbamylcholine (26 units), 27% excited and 19% inhibited; luteinizing hormone-releasing hormone (34 units), 38% excited and 12% inhibited; angiotensin II (ANG II) (34 units), 26% excited, and 6% inhibited; somatostatin (14 units), 36% excited and 43% inhibited; serotonin (15 units), 67% excited and none inhibited; and dopamine (13 units), 46% excited and none inhibited. Of the units tested, 86% lay less than 100 microns from the ventricular surface. Units responsive to ANG II lay deeper than units responding to serotonin (P less than 0.005), osmotic changes (P less than 0.001), or carbamylcholine (P less than 0.02).