Using perforated-patch voltage-clamp recording, a prominent hyperpolarization-activated inward cation current (Ih) has been identified in dissociated, cultured and replated, superior cervical sympathetic (SCG) neurones from 17-day-old rats. Ih was identified as a slowly activated inward current on hyperpolarizing from -60 mV, with an extrapolated null potential (in 3 mM [K+]out) of -42 mV. The activation range for Ih was -40 to -100 mV, with a half-activation voltage (V0.5) of -63 mV. The current was suppressed by 1 mM Cs+ but not by 1 mM Ba2+. The reversal potential for the current change induced by Cs+ agreed with the null potential for Ih. Ih conferred strong inward rectification to the current-voltage curve negative to -55 mV in both voltage-clamp and current-clamp recording. This inward rectification was reduced by 1 mM Cs+. In a sample of eight cells with initial resting membrane potentials between -51 and -64 mV, Cs+ increased the resting potential of all cells by between 2.5 and 21 mV. These results indicate that Ih contributes a tonic inward (depolarizing) component to the maintenance of the resting membrane potential in SCG neurones.