1. The effects of synaptic activation on rat supraoptic nucleus (s.o.n.) neurones were studied in the hypothalamic slice preparation. Intracellular recordings were obtained from forty-one probable magnocellular neuroendocrine cells using microelectrodes filled with 3 M-potassium acetate. Responses to single and repetitive stimulation of the area dorsolateral to the s.o.n., which would be expected to activate a cholinergic pathway (Hatton, Ho & Mason, 1983), were analysed. 2. In forty of forty-one cells, responses to single stimuli consisted of a short-latency excitatory post-synaptic potential (e.p.s.p.), which was often followed by a brief burst of fast depolarizing events which resembled spontaneous e.p.s.p.s. When the membrane was depolarized, single stimuli could consistently produce a burst of action potentials. 3. Brief trains of orthodromic stimuli produced three effects in most cells. Spontaneous fast depolarizing events, which appeared to be primarily e.p.s.p.s. significantly increased in frequency after the train. A slow membrane depolarization, which lasted up to 1-2 min, was observed in twenty-eight of forty-one cells. In several cells the slow depolarization was accompanied by an increase in input resistance (Ri). In some cells an after-discharge occurred during the slow depolarization. Slow depolarizations were observed in each of eleven phasic neurones, and in a smaller percentage of non-phasic and silent cells. 4. All components of the response to dorsolateral stimulation could be reduced or blocked in low-Ca2+, high-Mg2+ bathing medium. 5. Slow depolarizations were observed when action potentials were not elicited by the stimulus train. The slow depolarization was still present after manipulations that blocked discharge during the stimulus train, including injection of hyperpolarizing current and diffusion of the quaternary ammonium compound QX314. These data argue that the slow depolarization can occur independent of spike depolarizing after-potentials (d.a.p.s). 6. In some cells antidromic stimulation at an intensity just suprathreshold for the recorded cell did not produce comparable bursts of fast depolarizing events or slow depolarizations; similar periods of depolarizing current injection, which produced repetitive discharge, also did not mimic the effects of orthodromic stimulation. 7. The fast depolarizing events appear to reflect spontaneous e.p.s.p.s; increases in the frequency of these events may reflect the after-discharge of nearby neurones that are presynaptic to the recorded neurone.(ABSTRACT TRUNCATED AT 400 WORDS)