The effects of different pharmacological agents on the isometric developed tension (IDT) of right ventricles isolated from chronic chagasic C3HS mice were studied. The IDT in ventricles of chagasic animals was lower than in ventricles of normal mice (P < 0.01). Chagasic ventricles exhibited hyperreactivity to norepinephrine but not to epinephrine. This hyperreactivity was masked by propranolol and not by dibenamine. Acetylcholine produced a reduction of IDT to similar values in normal and chagasic myocardia. The possibility that the effect of the drugs tested might not result from specially beta adrenoceptor-mediated reactions, but from alterations at other cellular sites is discussed.