Whole cell and single-channel inwardly rectifying potassium currents (IK1) of freshly isolated single fetal (12 and 18 days) and neonatal (1, 5, and 10 days) rat ventricular myocytes were recorded using patch-clamp techniques. Whole cell IK1 was elicited by hyperpolarizing test pulses from a holding potential of -40 mV. IK1 densities increased markedly during heart development between fetal day 12 and until neonatal day 5; there was no further increase on neonatal day 10. Cell-attached patch recordings of single IK1 channels were employed with 150 mM K+ both in the pipette and bath solutions. Large-conductance (31 pS) K+ channels with short openings were observed both in fetal day 18 and neonatal day 5 but not in fetal day 12 cells. Only small-conductance (11 pS) channels with long openings were observed in approximately 10% of the cells on fetal day 12. The open probability of the large-conductance channels was 2.6-fold greater on neonatal day 5 than on fetal day 18. In conclusion, the increase in the IK1 density during observed heart development may be primarily due to an increase in the open probability of the IK1 channels, rather than to an increase in their number. We suggest that the small-conductance channel infrequently observed on fetal day 12 could be an immature type of IK1 channel, which disappears during development.