Angiotensin II receptors in adult guinea pig hearts were characterized using [125I][Sar1,Ile8]angiotensin II and the non-peptidic receptor antagonists, losartan and PD 123319. Autoradiographic experiments and binding experiments performed on membrane preparations showed that cardiac tissues mainly expressed losartan (Kd = 30 nM)-sensitive AT1 receptors. In contraction experiments, angiotensin II produced a positive inotropic effect in both right and left atrial preparations. This action was prevented by losartan but not by PD 123319. It is concluded that mainly AT1 receptors are expressed in guinea pig atria and that these sites are responsible for the positive inotropic effect of angiotensin II.