We used standard microelectrode techniques to study developmental changes in the actions of quinidine on the transmembrane potentials of Purkinje fibers obtained from the hearts of adult dogs and of dogs less than 1 month old. Quinidine had no major effect on resting membrane potential and action potential overshoot at either age. It reduced Vmax significantly at a basic cycle length of 500 msec, the magnitude of effect not differing with age. When frequency-dependent effects on Vmax were studied at basic cycle lengths of 1500 and 500 msec, the magnitude of the effect was not age-related nor were the time or number of beats to steady state effect and the recovery from quinidine effect. However, in its actions on repolarization, quinidine did demonstrate age dependence: the magnitude of change was greater in young than in adult fibers. Our results with quinidine are different from those of previous studies in which we showed age dependence of the effects of lidocaine on Vmax and repolarization, with greater effects occurring in the adult than in the neonate. This difference in the ability of lidocaine and of quinidine to modify transmembrane potential characteristics with age emphasizes the differences that exist in the effects of individual antiarrhythmic drugs on the determinants of cardiac electric activity.