The effects of higenamine were investigated in the single atrial and ventricular myocyte of the guinea pig by using patch clamp method. The results obtained were as follows: 1) Isoprenaline which is known to be β-agonist increased the duration of action potential and calcium current in ventricular cells. 2) Higenamine also increased the duration of action potential and calcium current in ventricular myocytes. And its effect was blocked by propranolol. 3) In the atrial cells, isoprenaline showed β-agonist effects, which were increasing the duration of action potential and calcium current same as in ventricular cells. 4) Higenamine, however, showed the opposite effects of β-agonist which were decreasing the duration of action potential and calcium current. The above results suggest that higenamine has the typical β-agonist effect in ventricular cells but inhibitory effect in atrial cells and this effect on atrium could be due to the reduction of calcium current.