Intracellular recordings were made from 168 sympathetic preganglionic neurons in the slice of the second or third thoracic spinal-cord segment of the adult cat to study the actions of noradrenaline on these neurons. Noradrenaline, applied by superfusion (0.5–50 μM), produced membrane depolarization in 73 neurons and membrane hyperpolarization in 39 neurons. In 26 neurons noradrenaline produced a biphasic response (depolarization–hyperpolarization or vice versa). The depolarization was blocked by prazosin, while the hyperpolarization was blocked by yohimbine. The noradrenaline-induced depolarization was associated with an increase in neuron input resistance, while the noradrenaline-induced hyperpolarization was associated with a decrease in neuron input resistance. Both responses decreased in amplitude with membrane hyperpolarization and were nullified at around the potassium equilibrium potential EK. The null potential of both responses became more and less negative with a decrease and an increase, respectively, in the extracellular potassium concentration. When the membrane potential was made more negative than EK, the noradrenaline-induced hyperpolarization reversed to depolarization in all cases, whereas in only 4 of 12 cases did the noradrenaline-induced depolarization reverse to hyperpolarization. These data suggest that the noradrenaline-induced depolarization is a result of a decrease, while the noradrenaline-induced hyperpolarization is a result of an increase in K+ conductance. Cobalt (2 mM), low calcium – high magnesium, and intracellular EGTA markedly reduced or abolished the noradrenaline-induced depolarization but had no significant effect on the noradrenaline-induced hyperpolarization. Barium (2 mM) depressed both responses. Tetraethylammonium (10–30 mM), 4-aminopyridine (3 mM), and cesium (2 mM) had no effect on either response. These data suggest that the noradrenaline-induced depolarization is a result of an inactivation of a background calcium-sensitive K+ conductance, while the noradrenaline-induced hyperpolarization is due to activation of a calcium-insensitive potassium conductance.Key words: K+ conductances, catecholamines, Ca2+ dependent, K+ current, spinal cord.