Celiprolol is a beta-adrenoceptor antagonist which has desirable ancillary properties since it is relatively cardioselective and can exert direct vasodilator and bronchodilator effects. Here agonist and antagonist effects of celiprolol at cardiac beta 1- and vascular beta 2-adrenoceptors were determined under in vivo conditions in the rat. All experiments were carried out in catecholamine-depleted, pentobarbital anesthetized and vagotomized rats, placed under artificial respiration. I.v. administrations were made via the femoral vein. Blood pressure was measured from the cannulated right carotid artery and heart rate was recorded with a cardiotachometer. Celiprolol (10 micrograms/kg to 1 mg/kg i.v.) produced dose-related increases in heart rate and decreases in mean carotid artery blood pressure which were of longer duration than those mediated by standard agonists of beta 1-(isoprenaline) or beta 2-(salbutamol) adrenoceptors respectively. Although the maximal increase in heart rate by celiprolol (110 +/- 4 beats/min, n = 7) was approximately half that of isoprenaline (198 +/- 1 beats/min, n = 5), isoprenaline acted at doses 200-fold lower than celiprolol. Betaxolol (0.03-0.3 mg/kg i.v.), a beta 1-adrenoceptor antagonist, inhibited strongly and with similar potency the tachycardiac effects of celiprolol (DR10 = 45 micrograms/kg i.v.) as well as isoprenaline (DR10 = 45 micrograms/kg i.v.). On the other hand, the hypotensive effects of celiprolol and salbutamol were antagonized markedly and with similar potency by ICI 118,551, a relatively selective beta 2-adrenoceptor antagonist (DR10 = 15 and 25 micrograms/kg i.v. respectively). In rats pretreated with celiprolol (0.03 to 0.3 mg/kg i.v.), the heart rate dose-response curves to isoprenaline were shifted to the right of those determined in matched groups of vehicle-pretreated animals. In this respect, celiprolol was half as potent as betaxolol in blocking cardiac beta 1-adrenoceptors. Furthermore, celiprolol also antagonized the hypotensive effects of salbutamol, but, in this respect, celiprolol was 90-fold less potent than ICI 118,551. In conclusion, these results clearly indicate that celiprolol has the ability of stimulating and blocking not only cardiac beta 1- but also vascular beta 2-adrenoceptors. The effects on cardiac beta 1-adrenoceptors as well as the agonism of beta 2-adrenoceptors are produced by similar doses of celiprolol. These doses are notably lower than those necessary to block beta 2-adrenoceptors. Thus, this pharmacological profile, which has also been demonstrated in humans, indicates that celiprolol is a modulator of cardiac beta 1-adrenoceptors with vascular beta 2-adrenoceptor agonist properties.