Whole-cell recordings were made from dissociated bullfrog sympathetic neurons to examine the actions of quinine (1-100 microM) on the steady-state activation and inactivation curves of a delayed rectifier-type potassium current (I(K)). Quinine (EC50 approximately 8 microM) caused a hyperpolarizing shift (approximately 31 mV with 30 microM) in the inactivation curve of I(K) without significantly affecting its activation curve. Quinine (20 microM) was without effects on the voltage-dependence of a rapidly-inactivating A-type potassium current (I(A)). It is concluded that quinine can selectively modulate the voltage-dependence of I(K) in amphibian autonomic neurons.