Intracellular recording techniques have been used to determine the electrophysiological properties of sympathetic neurones in ganglia of the caudal lumbar sympathetic chain (l.s.c.) and in the distal lobes of inferior mesenteric ganglia (i.m.g.) isolated from guinea-pigs. Passage of suprathreshold depolarizing current initiated transient bursts of action potentials in 97% of l.s.c. neurones, but only 13% of i.m.g. cells ('phasic' neurones). Most i.m.g. neurones fired continuously during prolonged depolarizing pulses ('tonic' neurones). Passive membrane properties varied; mean cell input resistance was similar between groups, but phasic neurones had smaller major input time constants on average than had tonic cells. Current-voltage relations determined under both current clamp and voltage clamp were linear around resting membrane potential (approximately 60 mV), where membrane conductance was lowest. Instantaneous and time-dependent rectification varied in the different neurone types. The current underlying the after-hyperpolarization following the action potential was significantly larger on average in tonic i.m.g. cells than in phasic neurones, although its time course (tau = 100 ms) was similar. Phasic neurones fired tonically when depolarized after adding the muscarinic agonist, bethanechol (10(-5) M to 10(-4) M), to the bathing solution. Bethanechol blocked a proportion of the maintained outward current (presumably the M-current, IM, Adams, Brown & Constanti, 1982) in phasic neurones; this current was small or absent in tonic neurones. Transient outward currents resembling the A-current (IA, Connor & Stevens, 1971 a) were evoked in tonic but not in phasic neurones by depolarization from resting membrane potential. IA could only be demonstrated in phasic neurones after a period of conditioning hyperpolarization. After a step depolarization to approximately --50 mV, IA reached peak amplitude at about 7 ms and then decayed with a time constant of about 25 ms in both neurone types. Activation characteristics of IA were similar for phasic and tonic neurones, but inactivation curves, although having the same shape, were shifted to more depolarized voltages in tonic neurones. That is, IA was largely inactivated at resting membrane potential in phasic, but not tonic neurones. It is concluded that the discharge patterns of the two populations of sympathetic neurones result from differences in the voltage-dependent potassium channels present in their membranes. The anatomical occurrence of the different cell types suggests that phasic neurones are vasoconstrictor and tonic neurones are involved with visceral motility.