The effect of catecholamine depletion (induced by prior treatment with reserpine) was studied in Wistar rat ventricular myocytes, using whole-cell voltage-clamp methods. Two calcium-independent outward currents, the transient I_to and the sustained I_sus currents were measured. Reserpine treatment decreased tissue noradrenaline content by 97%. Action potential duration, in isolated perfused heart, was significantly increased in reserpine-treated hearts. In isolated ventricular myocytes, I_to density was decreased by 51% in reserpine-treated rats. This treatment had no effect on I_sus. I_to steady-state inactivation-voltage relationship and recovery from inactivation remained unchanged whereas the conductance-voltage activation curve for reserpine-treated rats was significantly shifted (6.7 mV) towards negative potentials. The incubation of myocytes with 10 µM noradrenamine for 7 to 10 h restored Ito, an effect abolished by the presence of actinomycin D.0.5 µM noradrenaline had no effect on I_to. However in the presence of both 0.5µM noradrenaline and neuropeptide Y (0.1 µM) Ito density was restored to its control value. These results suggest that the sympathetic nervous system is involved in Ito regulation. Sympathetic noradrenaline depletion decreased the number of functional channels via an effect on the -adrenergic cascade and noradrenaline is able to restore expression of I_to channels.