Studies on cardiac cell response to electric field stimulation are important for understanding basic phenomena underlying cardiac defibrillation, In this work, we used a model of a prolate spheroidal cell in a uniform external field (Klee and Plonsey, 1976) to predict the threshold electric field (E-T) for stimulation of isolated ventricular myocytes of rats at different ages. The model assumes that E-T is primarily determined by cell shape and dimensions, which markedly change during postnatal development, Neonatal cells showed very high E-T, which progressively decreased with maturation (experimental mean values were 29, 21, 13, and 5.9 and 6.3 V/cm for 3-6, 13-16, 20-21, 28-35, and 120-180 day-old rats, respectively, P < 0.001; theoretical values were 24, 18, 11, 9, and 6 V/cm, respectively). Estimated maximum membrane depolarization at threshold (<Delta>V-T approximate to 35 mV, under our experimental conditions) was reasonably constant during development, except for cells from 1-mo-old animals, in which DeltaV(T) was lower than at other ages. We conclude that the model reasonably correlates E-T with cell geometry and size in most cases, Our results might be relevant for the development of efficient procedures for defibrillation of pediatric patients.486630636