In this study, the effect of staurosporine, a potent protein kinase C (PKC) inhibitor, on Na + current ( I Na) was examined by whole-cell patch recording in rabbit atrial myocytes. The most prominent staurosporine effect was a slowing of I Na inactivation and 1 μM staurosporine reduced amplitude of I Na about 33%. Staurosporine decreased I Na at all potentials and slowed the I Na inactivation in a dose-dependent manner, with a K d value of 1.107 ± 0.162 μM. Staurosporine did not change the recovery kinetics and show use dependence. However, the activation and the steady-state inactivation curves were shifted toward more negative potentials (− 5.5 and − 5.1 mV, respectively). Two other PKC inhibitors, GF 109203X (1 μM) and chelerythrine (3 μM), did not show a slowing effect on I Na inactivation. In conclusion, our results indicate that the slowing of I Na inactivation by staurosporine seems not to be through blockade of PKC rather to act directly on the Na + channels, and the direct blocking effects of staurosporine on the Na + channel should be taken into consideration when staurosporine is used in functional studies of ion channel modulation by protein phosphorylation.