The functional expression of A-type K⁺ channels (I_A) was examined in chick lumbar motoneurons (LMNs) at embryonic days 6 and 11 (E6 and E11). We observed a three-fold increase in I_A density between E6 and E11 in spinal cord slices and acutely dissociated LMNs. There was no change in current density, kinetics, or voltage-dependence of I_A in E11 homozygous limbless mutants or in E11 embryos in which hindlimbs were surgically removed at E6. Moreover, chronic in ovo administration of D-tubocurarine, which causes an increase in motoneuron branching on the surface of target muscles, had no effect on I_A. Electrical activity played an important role in I_A regulation in LMNs in vitro and in ovo. Blocking spontaneous electrical activity of LMNs by chronic in ovo application of mecamylamine or muscimol reduced I_A by 80%. LMNs cultured in the presence of tetrodotoxin also failed to express normal densities of I_A, even when the cultures also contained target tissues. The portion of I_A that remained after in ovo or in vitro blockade of activity inactivated more quickly than the I_A of LMNs that were allowed to discharge spikes. The developmental expression of LMN I_A increases significantly during development and this increase is activity-dependent but does not require interactions with target tissues. Ongoing activity also appears to regulate the kinetics of I_A inactivation.