We have compared transsarcolemmal Ca2+ currents in acutely isolated neonatal (1- to 5-d-old) and adult rabbit cardiac myocytes prepared using similar enzymatic techniques. Time- and voltage-dependent inward Ca2+ currents were measured using the whole-cell voltage clamp technique. In neonatal myocytes, peak Ca2+ currents measured 114 +/- 10 pA (mean +/- SEM, n = 18) as compared with 2014 +/- 403 pA in adult myocytes (n = 5, p less than 0.001). Although adult myocytes had a larger surface area (estimated from cell capacitance) than neonatal cells (113 +/- 15 x 10(-6) versus 28 +/- 2 x 10(-6) cm2, p less than 0.001), the calculated peak current density was also significantly larger in adult cells (17.9 +/- 2.5 compared to 4.3 +/- 0.4 microA/cm2 for neonatal cells, p less than 0.001). The voltage dependence of the peak Ca2+ current was similar in neonatal and adult myocytes. Early transient (T-type) Ca2+ currents were also studied by comparing the current induced by depolarization to -20 mV from holding potentials of -40 and -80 mV. T-type Ca2+ channels were present in 91% of the adult cells but were evident in only 39% of the neonatal cells. In summary, voltage-gated Ca2+ current amplitude, current density, and T-type Ca2+ channel prevalence all increase with maturation. These data suggest that neonatal myocytes may be relatively deficient in Ca2+ channel activity when compared to adult myocytes.