1. Intracellular electrical recording techniques were used to study the ionic mechanisms of cholinergic synaptic transmission in cat vesical pelvic ganglia (v.p.g.). 2. Orthodromic nerve stimulation as well as ionophoretic application of acetylcholine (ACh) resulted in, first, a fast excitatory post-synaptic potential (f.e.p.s.p.) and secondly, a slow inhibitory post-synaptic potential (s.i.p.s.p). These distinct post-synaptic responses were direct actions of ACh and not mediated through an interneurone. In addition, a slow excitatory post-synaptic potential (s.e.p.s.p.) was observed in 44% of the cells. 3. The f.e.p.s.p., mediated via nicotinic receptors, had a reversal potential of -10 mV and resembled the conventional rapid depolarization in other ganglia. The s.i.p.s.p., mediated by muscarinic receptors, had a reversal potential of about -100 mV and resulted from an increase in potassium conductance. 4. The slow muscarinic hyperpolarization could be observed in the absence of antagonists and it was elicited at stimulus frequencies in the physiological range (2-10 Hz). the s.i.p.s.p. induced orthodromically or ionophoretically inhibited firing in spontaneously active neurones. These observations suggest that the muscarinic hyperpolarization may occur under physiological conditions and has sufficient magnitude to be inhibitory to neuronal activity.