The pharmacology of celiprolol and its relationship to some other beta-adrenoceptor antagonists is described. Celiprolol is a potent beta blocker on beta 1-adrenoceptors and exhibits cardioselectivity both in vitro and in vivo in the pithed rat, but shows no significant in vitro alpha 1-blocking action. Celiprolol differs from atenolol in that it has an intrinsic sympathomimetic activity (ISA) for beta 1-adrenoceptors, which is reflected in its relative lack of negative inotropic effects in humans. In the pithed rat, celiprolol's ISA was demonstrated at much lower doses than for pindolol, even though pindolol has a similar potency to celiprolol in antagonizing the heart rate effects of isoproterenol. It was completely blocked by propranolol, indicating that celiprolol behaves like a partial agonist for beta 1-adrenoceptors, whereas the ISA developed by pindolol was only partially blocked by propranolol. These data suggest a different mechanism for the development of ISA between celiprolol and pindolol.