The effects of injections of monoamines, alone and in combination with different antagonists, bilaterally into the nucleus accumbens of nialamide-pretreated rats were investigated. Dopamine was found to produce a stronger stimulation of locomotor activity than noradrenaline, whereas serotonin was effective only in a small number of animals, in which the duration of locomotor stimulation was shorter than after dopamine or noradrenaline. The effects of both dopamine and noradrenaline were completely antagonized by administration of a small dose of the dopamine antagonist haloperidol, administered bilaterally 15 min after the catecholamines. The alpha-adrenergic antagonist phentolamine did not inhibit the effect of noradrenaline but, on the contrary potentiated and considerably prolonged the duration of locomotor stimulation. Aslo, the effect of dopamine was potentiated and prolonged by phentolamine. Bilateral injection of phentolamine alone had no influence upon locomotor activity. The effect of noradrenaline was not clearly inhibited nor potentiated by the beta-adrenergic antagonist propranolol. It is suggested that the stimulation of locomotor activity induced by injection of noradrenaline into the uncleus accumbens of nialamide-pretreated rats is brought about via dopaminergic mechanisms.