Identification and developmental changes in the transient outward current (I(to)) in isolated embryonic chick ventricular cells (3, 10 and 17 days old) were examined using a whole-cell voltage clamp technique. Experiments were performed at room temperature (22 degrees C). Test pulses were applied between -40 and +50 mV from a holding potential of -60 mV. The I(to) was present (but small) and increased during development; the current density of I(to) at +40 mV was 3.5 +/- 0.5 pA/pF (n = 7) in 3-day cells, 4.2 +/- 0.9 pA/pF (n = 5) in 10-day cells, and 17.1 +/- 1.6 pA/pF (n = 5) in 17-day-old cells. The average capacitances also changed with developmental age; 12.0 +/- 2.0 pF (n = 8) in 3-day cells, 10.8 +/- 2.2 pF (n = 7) in 10-day cells, and 8.6 +/- 2.3 pF (n = 7) in 17-day cells. The I(to) was not always observed in all the prepared cells, and the number of cells possessing I(to) increased during development. The threshold potential was -30 mV in 17-day cells, and appeared to be displaced to more negative potential with developmental age. The time to peak decreased during development: 10.6 +/- 1.1 ms (n = 4) in 3-day cells, 6.7 +/- 0.5 ms (n = 5) in 10-day cells, and 5.4 +/- 0.6 ms (n = 5) in 17-day cells. The time decay of the inactivation phase for the I(to) had two exponentials; the fast component was increased by about 3-fold in 17-day cells, and the slow component was decreased by about 14% in both 10- and 17-day cells, as compared to 3-day cells. Addition of 3 mM 4-aminopyridine (4-AP) inhibited I(to) at +50 mV by 81.9 +/- 2.3% (n = 4, P < 0.001). These results indicate that the I(to), voltage-dependent and 4-AP-sensitive, exists even in young embryonic cardiomyocytes (but not in all cells), and increases during development, resulting in modulation of the action potential configuration.