To determine whether an increase in plasma volume might directly influence supraoptic neurones, single cell extracellular recordings were made from magnocellular neurones of the supraoptic nucleus in urethane-anaesthetized rats as plasma volume was expanded by intragastric injection of isotonic saline. Continuous ratemeter records taken before, during and after intragastric injections of 10 ml isotonic saline showed that the firing rate of putative vasopressin cells was reduced by 2.21 spikes/s (P < 0.02; n = 9; paired t-test) after 50 min. Putative oxytocin cells, after an initial increase in firing rate which lasted approximately 30 min, showed a decrease of 0.98 spikes/s (P < 0.02; n = 6; paired t-test). A population of 93 control cells of both types had a median firing rate of 4.69 spikes/s, a comparable group of 65 cells recorded 1 h after intragastric injection had a median firing rate of 3.15 spikes/s and another group of 68 cells recorded 1 h after a second injection had a median firing rate of 2.5 spikes/s. These differences were significant (P < 0.04 and P < 0.01; Mann-Whitney U test). The haematocrit of plasma samples taken from five similarly anaesthetized control animals was 49.7%. One hour after one intragastric injection the value was significantly (P < 0.02; paired t-test) reduced to 46.7% and 1 h after a second injection it was further reduced to 42.1% (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)