The mechanism for the reduction of the transient outward K⁺ current (I_to) in diseased myocardium is unknown. To identify potential mechanisms, the reduction of I_to and its subsequent restoration by norepinephrine (NE) were studied in cultured canine epicardial myocytes. After myocytes were cultured for 9 days (day 9), I_to density was decreased compared with density on the day of isolation (day 0) (3.2 ± 0.4 vs. 10.4 ± 0.4 pA/pF; mean ± SE). The time constant of current decay (_decay) was increased, the time course of recovery from inactivation was prolonged, and the half-inactivation voltage (V_1/2) was shifted to less negative potentials. Exposure of myocytes on day 8 to 1 µM NE or isoproterenol (Iso) for 1 h had no acute effect on I_to but restored I_to density to 7.6 ± 1.2 or 9.7 ± 2.3 pA/pF, respectively, on day 9. Recovery from inactivation and _decay remained slowed, and V_1/2 remained shifted to less negative potentials. The effects of NE and Iso were blocked by actinomycin D and were not mimicked by phenylephrine or phorbol ester. A-23187 (1 µM) also restored I_to. Thus -adrenergic agonists restored normal I_to density, but not normal I_to kinetics, in cultured epicardial myocytes, possibly via increased intracellular Ca²⁺ concentration.