Whole cell, Ca2+ -independent, depolarization-activated K+ currents were characterized in identified callosal-projecting (CP) neurons isolated from postnatal day 7-16 rat primary visual cortex. CP neurons were identified in vitro after in vivo retrograde labeling with fluorescently tagged latex microbeads. During brief (160-ms) depolarizing voltage steps to potentials between -50 and +60 mV, outward K+ currents in these cells activate rapidly and inactivate to varying degrees. Three distinct K+ currents were separated based on differential sensitivity to 4-aminopyridine (4-AP); these are referred to here as I(A), I(D), and I(K), because their properties are similar (but not identical) K+ currents termed I(A), I(D), and I(K) in other cells. The current sensitive to high (≤100 μM) concentrations of 4-AP (I(A)) activates and inactivates rapidly; the current blocked completely by low (≤50 μM) 4-AP (I(D)) activates rapidly and inactivates slowly. A slowly activating, slowly inactivating current (I(K)) remains in the presence of 5 m.M 4-AP. 1(A), I(D), and I(K) also were separated and characterized in experiments that did not rely on the use of 4-AP. All CP cells express all three K+ current types, although the relative densities of I(A), I(D), and I(K) vary among cells. The experiments here also have revealed that I(A), I(D), and I(K) display similar voltage dependences of activation and steady state inactivation, whereas the kinetic properties of the currents are distinct. At +30 mV, for example, mean ± SD activation τs are 0.83 ± 0.24 ms for I(A), 1.74 ± 0.49 ms for I(D), and 14.7 ± 4.0 ms for I(K). Mean ± SD inactivation τs for I(A) and I(D) are 26 ± 7 ms and 569 ± 143 ms, respectively. Inactivation of I(K) is biexponential with mean ± SD inactivation time constants of 475 ± 232 ms and 3.128 ± 1.328 ms: ~20% of the 4-AP-insensitive current is noninactivating. For all three components, activation is voltage dependent, increasing with increasing depolarization, whereas inactivation is voltage independent. Both I(A) and I(K) recover rapidly from steady state inactivation with mean ± SD recovery time constants of 38 ± 7 ms and 79 ± 26 ms, respectively; I(D) recovers an order of magnitude more slowly (588 ± 274 ms). The properties of I(A), I(D), and I(K) in CP neurons are compared with those of similar currents described previously in other mammalian central neurons and, in the accompanying paper, the roles of these conductances in regulating the tiring properties of CP neurons are explored.