The influence of beta-adrenoceptor antagonists on the spontaneous rate and on force of contraction of the myocardium was studied independently on spontaneously beating right and electrically driven isolated left atria of rabbit. On spontaneous rate, practolol had sympathomimetic effect only, N-isopropylmethoxamine (IMA), had both sympathomimetic as well as depressant effects, whereas alprenolol, procinolol, bunolol and H 35/25 had depressant effects only in higher concentrations. The order of potency was procinolol greater than bunolol greater than alprenolol greater than H 35/25 greater than and IMA. On the contractions of isolated left atria, all beta-adrenoceptor antagonists produced concentration-dependent depressant effect. In relation to procinolol, these agents were 5-125 times less potent for depressing the contractions of left atria by 15% and the order of beta-potency was procinolol greater than alprenolol greater than bunolol = H 35/25 greater than IMA greater than and practolol. The present results indicate that the depressant effects of beta-adrenoceptor antagonists on spontaneous rate of right atria and on contraction of isolated left atria are not related to each other.