Three β-adrenoceptor antagonists—atenolol (A), metoprolol (M), and pamatolol (P)—were examined with respect to their effects on iso-proterenol responses of isolated right atria and portal veins from rats. With all three blockers at concentrations of 10-7, 10-6, and 10-5 the chronotropic atrial responses indicated a linear relation with slope of 1 for log (dose ratio - 1) versus log[blocker]. The following pA2 values were obtained: A = 7.14, M = 7.51, P = 7.56. Inhibition of spontaneous contractions in portal veins by isoproterenol was studied after α-adrenoceptor blockade by phenoxybenzamine. The β-adrenoceptor antagonists were used at 10-6, 10-5, and 10-5 M in these experiments. Only with P did the relation log (dose ratio-1) versus log[blocker] show a slope near unity, whereas A and M gave slopes significantly less than 1. Intercepts of the regression lines with the abscissa corresponded to pA2 values of A = 6.01, M = 6.51, P = 5.54. The results indicate that all three substances are “cardioselective” β-adrenoceptor antagonists devoid of intrinsic β-stimulatory action. A and M show a comparable degree of cardioselectivity, whereas P is more selective.