Antiarrhythmic efficacy of nicorandil (SG-75) (1-100 microM), a coronary vasodilator, was investigated electrophysiologically with regard to the action potentials of canine Purkinje fibers. The main results obtained are as follows: (a) nicorandil suppressed three kinds of automaticities, i.e., spontaneous or low-K+-induced automaticity and electrical depolarization-induced automaticity; (b) the drug increased the effective refractory period relative to action potential duration and decreased differences between action potential duration and effective refractory period; and (c) in the presence of nicorandil (50 microM), membrane potentials at which the earliest premature response could be elicited were significantly more negative than control, thus leading to a much faster upstroke velocity of the premature response. These changes in electrophysiological properties suggest that the drug may be effective for treating selected cardiac arrhythmias due to both enhanced automaticity and reentry, particularly in the presence of a decreased membrane K conductance. These effects of nicorandil could be mostly attributed to an increase in membrane K conductance.