In anaesthetized dogs and guinea pigs effects of carbocromen on the specific conduction system of the heart (SA- and AV-node) are demonstrated using a special experimental model. 2. Carbocromen stimulates and reestablishes to normal SA- and AV-node activity previously depressed by organic Ca2+-antagonistic drugs. 3. The results suggest an action of carbocromen on the "slow membrane currents" because it antagonizes negative effects of slow current inhibitors. There is no indication that the drug acts as an Ca2+-agonist. The basic mechanism of action is not clear, different possible effects are discussed. 4. Practical clinical consequences are reviewed: by improving a depressed AV-conduction carbocromen may prevent or interrupt different junctional re-entry arrhythmias.
