Our aim was to determine whether the expression of K⁺ currents is related to the cell cycle in the excitable GH3 pituitary cell line. K⁺ currents were studied by electrophysiology, and bromodeoxyuridine (BrdU) labeling was used to compare their expression in cells thereafter identified as being in the S or non-S phase of the cell cycle. We show that the peak density of the transient outward K⁺ current (I_to) was 33% lower in cells in S phase (BrdU+) than in cells in other phases of the cell cycle (BrdU). The voltage-dependence of I_to was not modified. However, of the two kinetic components of I_to inactivation, the characteristics of the fast component differed significantly between BrdU+ and BrdU cells. Recovery from inactivation of I_to showed biexponential and monoexponential function in BrdU and BrdU+ cells, respectively. This suggests that the molecular basis of this current varies during the cell cycle. We further demonstrated that 4-aminopyridine, which blocks I_to, inhibited GH3 cell proliferation without altering the membrane potential. These data suggest that I_to may play a role in GH3 cell proliferation processes.