1. The influence of antifibrillatory agents on the release of previously accumulated (86)Rb(+) ions was studied in guinea-pig isolated left auricles, incubated in a K(+)-free Tyrode solution that contained an equivalent concentration (2.7 mM) of Rb(+). The auricles were stimulated electrically at a frequency of 100/min and showed entirely "normal" mechanical behaviour in the incubation medium.2. Quinidine, tetracaine, hexobarbitone-Na and propranolol caused a dose-dependent negative inotropic effect. The frequency of beating (100/min) was usually not affected by these drugs.3. The reduction in contractile force was accompanied by a dose-dependent inhibition of (86)Rb release. The concentration-response curves for both the mechanical effect and the influence on (86)Rb release almost coincided for quinidine, although these curves diverged for the other drugs studied.4. It is suggested that all the four different antifibrillatory drugs cause a general reduction of the membrane permeability to inorganic ions. The mechanism of this membrane stabilization remains unknown.