Using perforated-patch voltage-clamp recording, a prominent hyperpolarization-activated inward cation current (I _h) has been identified in dissociated, cultured and replated, superior cervical sympathetic (SCG) neurones from 17-day-old rats. I _h 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 I _h was –40 to –100 mV, with a half-activation voltage (V _0.5) 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 I _h. I _h 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 I _h contributes a tonic inward (depolarizing) component to the maintenance of the resting membrane potential in SCG neurones.