The effects of acetylation of sympathomimetic amines, tyramine, amphetamine, ephedrine, phenylephrine, orciprenaline, and salbutamol, and their O- and N-acetyl derivatives and the effects of reserpine or physostigmine pretreatment on the isolated auricles and tracheal chain of guinea-pigs have been studied. All the parent drugs relaxed the tracheal chain and had a positive inotropic and chronotropic effect on the isolated auricles; only amphetamine, on the contrary, contracted the tracheal chain. O-acetylation of these sympathomimetic amines generally decreased less chronotropic than iontropic action on the isolated auricles. O-acetylation of tyramine however: actually increased the positive chronotropic activity of drug. As a rule, O-acetylation also decreased the beta-adrenergic effect of these compounds on the tracheal chain, but not so markedly as on the isolated auricles. N-acetylation generally abolished the adrenergic effects of these sympathomimetic amines on the isolated auricles and decreased those effects on the tracheal preparation. N,O-triacetylation of salbutamol abolished the stimulating effect of the parent drug on the auricles but increased the relaxant activity on the trachea. Physostigmine antagonized the effects of O-acetyltyramine and O-triacetylorciprenaline but not those of tyramine and orciprenaline on the trachea preparation. It is concluded that among the sympathomimetic amines acetylation may be utilized for the development of specific bronchodilators and O-acetylation for inducing drug latentiation.