The excitability of various neurones in the mammalian central nervous system (CNS), ranging from motoneurones to serotonergic neurones, is enhanced by alpha 1-adrenoceptor agonists. Excitations mediated via alpha 1-adrenoceptors are associated with a slow depolarization and an increase in input resistance, probably resulting from a decrease in resting potassium conductance. However, the involvement of voltage-dependent transient currents in mediating alpha 1 excitatory effects has not been evaluated. An early transient outward current has been described which is important in regulating the frequency of repetitive firing; it is activated by depolarizing voltage steps from potentials more negative than rest and blocked by 4-aminopyridine. This current, which has been termed 'IA', was found originally in invertebrates and subsequently in various vertebrate neurones. The present single-electrode voltage-clamp study demonstrates an early transient outward current (IA) in serotonergic neurones which is suppressed by noradrenaline and the alpha 1-agonist phenylephrine; a suppression of IA may account in part for the acceleration of pacemaker activity induced by alpha 1-agonists in serotonergic neurones.