The effects of specific CaM kinase II inhibitors were investigated on Na+ channels from rat cerebellar granule tells. A maximal effect of KN-62 was observed at 20 mu M and consisted of an 80% reduction of the peak Na+ current after only a 10-min application. A hyperpolarizing shift of 8 mV in the steady-state inactivation was also observed. KN-04 (20 mu M), an inactive analog, had no detectable effect. KN-62 was however inactive on Na+ currents recorded from Chinese hamster ovary cells expressing the type II A alpha subunit. We have also analyzed the inhibitory effects of CaM kinase II 296-311 and CaM kinase II 281-309 peptides. Both peptides (75 mu M) induced a maximum peak Na+ current reduction within 30 min. Under similar conditions, a truncated peptide CaM kinase II 284-302 was ineffective. These results demonstrate that CaM;I kinase II acts as a modulator of Na+ channel activity in cerebellar granule cells.