Extracellular action potentials were recorded from the organum vasculosum laminae terminalis (OVLT) in rat brain slice preparations; the effect of different concentrations of NaCl on spontaneous firing frequency was studied. From 72 neurons, 67 (93%) were responsive to various perfusion media, while 5 neurons (7%) were not responsive. The change from the standard medium (SM; 124 mM NaCl) to 99 mM, decreased the firing frequency in 24 (65%) and increased it in 13 (35%) out of 37 responsive cells. The change from the SM to 149 mM evoked an increase in the firing rate in 33 (73%) and a decrease in 12 (27%) of 45 responsive neurons; the change to 174 mM, increased the firing rate in 5 (100%) neurons tested. The excitatory effect of increasing [NaCl] in the perfusion medium persisted even in low Ca2+ and high Mg2+ medium. Mannitol (55 mM) added to the SM increased the firing rate of cells; no significant decrease in the firing rate was seen with sodium mannitol 99/55 mM. Ouabain (OUA) (0.1 x 10(-3) mM) added to the SM increased the firing rate in 16 (84%) and decreased it in 3 (16%) out of 19 cells. Diphenylhydantoin (DPH) (1 mM) added to the SM decreased the firing rate in 12 (67%) and increased it in 6 (33%) of 18 cells tested. Hypo- or hypertonic NaCl solutions had no consistent effect on the spontaneous activity of 14 pyramidal cells recorded from the hippocampus (area CA3). The results emphasize the importance of intracellular Na content as a physiological trigger regulating the activity in neurons of the OVLT.