1. Intracellular recordings were made from rat sympathetic neurones in isolated superior cervical ganglia (SCG), coeliac ganglia (CG) and superior mesenteric ganglia (SMG). 2. Based on their response to a maintained depolarizing current stimulus, neurones were classified as 'phasic' or 'tonic'. All neurones in the SCG were phasic, 85% of the neurones in the SMG and 58% of the neurones in the CG were tonic, and the remainder were phasic. 3. The voltage response of phasic and tonic neurones around threshold to a constant current step was markedly different. The response of phasic neurones was biphasic with an initial depolarizing response followed by significant repolarization of the membrane potential. In contrast, tonic neurones became more depolarized during a prolonged current step. 4. The underlying currents were studied using single-electrode voltage-clamp recording. The M-current was present in all phasic neurones, but was very weak or absent in tonic neurones. 5. An A-current was apparent in both phasic and tonic neurones. The voltage-dependent activation, steady- state inactivation, and current density of the A-current were all similar in phasic and tonic cells. 6. A low- threshold, slowly inactivating outward current (D2-current) was observed exclusively in tonic neurones. The slow inactivation of this current appeared to underlie the slow depolarizing ramp seen in response to a maintained depolarizing current step. 7. Computer simulations, based on the voltage-clamp data, suggested that the different firing properties of phasic and tonic neurones could be accounted for by differential expression of the M-current.