The i.p. injection of apomorphine (APO) or N-n-propyl-norapomorphine (NPA) to adult male rats markedly increases the incidence of episodes of penile erection (PE). The effect is dose related for a dose range of APO from 25-500 .mu.g/kg and NPA from 1.25-50 .mu.g/kg. Higher doses are progressively less effective until 1 and 5 mg/kg of APO and NPA, respectively, suppress even the natural occurrence of PE. APO differs from NPA not only on a potency basis but also in that doses of APO which elicit stereotypy no longer evoke PE, while a wide range of doses of NPA (12.5-500 .mu.g/kg) produces simultaneously strong PE activation and stereotypy. The sexual stimulant response to APO and NPA is prevented by haloperidol (0.3 mg/kg) and sulpiride (50 mg/kg) but is not affected by domperidone (3 mg/kg), an inhibitor of extracerebral dopamine receptors.