1. Intracellular recordings were obtained from neurones of the tuberomammillary nucleus in an in vitro explant of the rat hypothalamus. 2. Tuberomammillary neurones were spontaneously active (2.1 +/- 0.6 Hz) at the resting potential which was around -50 mV. Action potential amplitude was 75 +/- 8 mV (n = 9); mean mid-amplitude duration was 1.8 +/- 0.4 ms (n = 9). 3. The mean input resistance of tuberomammillary neurones was 176 +/- 42 M omega (n = 30), and the mean membrane time constant was 19.8 +/- 5.3 ms (n = 30). These neurones exhibited inward rectification with hyperpolarization from the resting potential, and transient outward rectification at the offset of hyperpolarizing electrotonic pulses. 4. Action potentials were followed by an after-hyperpolarization of 300-600 ms duration and 12-18 mV amplitude. This after-hyperpolarization had a reversal potential around -80 mV, was abolished by intracellular loading with caesium, and was reduced but not abolished by bath application of either cadmium, cobalt or nickel. 5. Tetrodotoxin abolished spontaneous action potentials. Further addition of tetraethylammonium ions revealed a regenerative spike which was reversibly blocked by the addition of cobalt. 6. That tuberomammillary neurones exhibiting these properties were indeed histaminergic was confirmed in five cases by intracellular ionophoresis of Lucifer Yellow and subsequent double labelling by immunofluorescent localization of the histamine synthetic enzyme L-histidine decarboxylase.