1. The effects of carazolol, celiprolol and their respective fluorinated derivatives (FD) on electrical stimulation threshold (EST) and ventricular fibrillation threshold (VFT) were compared with those of propranolol and propranolol-FD in Langendorff-perfused rabbit hearts. 2. Carazolol, carazolol-FD, propranolol and propranolol-FD produced significant dose-dependent elevation of both EST and VFT at all tested concentration levels. In contrast, celiprolol and celiprolol-FD did not produce a significant change in either threshold. 3. On a dosage basis, the order of antifibrillatory potency of these compounds is: carazolol-FD > propranolol-FD > carazolol > propranolol > celiprolol-FD > celiprolol. 4. The results of this study seem to indicate the importance of membrane stabilizing effect for a potent antifibrillatory action of beta-adrenoceptor blocking agents. Furthermore, fluorination is demonstrated to produce more potent antifibrillatory activity than that of the parent drugs.