Ca2+/calmodulin-dependent protein kinase II contains two subunits, alpha (Mr 50,000) and beta (Mr 60,000/58,000), both of which undergo Ca2+/calmodulin-dependent autophosphorylation. In the present study, we have studied the mechanism of this autophosphorylation reaction and its effect on the activity of the enzyme. Both subunits are autophosphorylated through an intramolecular mechanism. Using synapsin I as substrate, Ca2+/calmodulin-dependent protein kinase II, in its unphosphorylated form, was totally dependent on Ca2+ and calmodulin for its activity. Preincubation of the enzyme with Ca2+, calmodulin, and ATP, under conditions where autophosphorylation of both subunits occurred, converted the enzyme to one that was only partially dependent on Ca2+ and calmodulin for its activity. No change in the total activity, measured in the presence of Ca2+ and calmodulin, was observed. The nonhydrolyzable ATP analog adenosine 5'-[beta, gamma-imido] triphosphate did not substitute for ATP in the preincubation. Moreover, dephosphorylation of autophosphorylated Ca2+/calmodulin-dependent protein kinase II with protein phosphatase 2A resulted in an enzyme that was again totally dependent on Ca2+ and calmodulin for its activity. We propose that autophosphorylation and dephosphorylation reversibly regulate the Ca2+ and calmodulin requirement of Ca2+/calmodulin-dependent protein kinase II.