SUMMARY1. Cholic acid has a dose‐dependent negative chronotropic effect on isolated atria ofWistar rats.2. The positive inotropic effect of cholic acid is the result of a negative chronotropic effect and can be eliminated by electrical pacing.3. Cholic acid does not appear to exert its negative chronotropic effect through cholinoceptors and alterations in bath concentrations of calcium and potassium does not influence this effect significantly.4. Cholic acid is a functional antagonist of isoprenaline.5. It is suggested that cholic acid exerts its negative chronotropic effect by forming a monolayer on the surface of the cell membrane, thereby mechanically interfering with membrane functio