A number of centrally active drugs were tested for antagonism of physostigmine- or apomorphine-induced yawning and for apomorphine-induced penile erections. The alpha 2-adrenoceptor antagonists piperoxan and idazoxan inhibited the yawning response without affecting the penile erections. The 5HT agonist quipazine and the histamine antagonist dexchlorpheniramine inhibited the yawning response more effectively than the penile erections. Dexchlorpheniramine even enhanced the apomorphine-induced penile erections and induced penile erections in physostigmine-treated rats. The 5HT antagonists metergoline and methysergide blocked the apomorphine-induced penile erections without affecting the yawning response. The alpha 2-adrenoceptor agonist clonidine, the dopamine antagonist sulpiride, the antihistaminic mepyramine and the benzodiazepine chlordiazepoxide inhibited both yawning and penile erections at the same dose level. The alpha 1-adrenoceptor antagonists prazosin and phenoxybenzamine were inactive. It is concluded that yawning and penile erections can be differentially affected by drug treatments. Also, while concomitant yawning and penile erections can be selectively induced by a class of dopamine receptor agonists, the same selectivity does not apply to antagonism of these induced behaviours.