hole-cell calcium current (ICa) and inwardly rectifying potassium current (IK1) were studied in 21-day fetal, 28-day fetal (total gestation, 31 days), 2–5-day neonatal, and adult rabbit ventricular myocytes isolated by enzymatic dissociation. Whole-cell peak ICaand IK1at −100 mV increased significantly after birth. Cell size approximated from cell membrane capacitance also increased with age, with the most significant increase occurring after birth. When normalized to cell surface area, peak ICa. density increased from day 21 of gestation to the neonate and then increased again from neonate to adult. In all age groups, peak ICaoccurred at a test potential of +10 mV, and the shape of the Ca2+current-voltage relation did not change with age. These findings suggest that there are no significant developmental changes in the voltage dependence of ICa-Therefore, the measured age-related increase in Ca2+current density may result from increased channel expression. IK1also exhibited a pattern of increasing current density with age. For IK1, the increase in current density was most rapid between day 21 and the perinatal period and much slower after birth. These results demonstrate that ICaand IK1undergo significant changes during late fetal and postnatal development. (Circulation Research1992;70:508–515)