This article is illustrative of how proteins phosphorylate/dephosphorylate in vivo. The authors utilized a-toxin-permeabilized bTC3 cell for the identification of protein phosphatases responsible for the dephosphorylation and deactivation of calmodulin-dependent protein kinase II (CaM kinase II) in situ.They show that an increase in calmodulin from .05 to 10 mM induced the almost-total conversion of CaM kinase II into a calmodulin-independent form typical of autophosphorylated, activated enzyme. Removing calmodulin caused CaM Kinase II to quickly go back to prestimulated levels. This observed reversal was diminished, but not prevented, by the action of inhibitors of protein phosphatase-1 (PP-1) and PP-2A, okadaic acid and calyculin A, and also by the selective chelation of Mg++ upon addition of EDTA. When both okadaic acid and EDTA were present one can observe arrest of enzyme deactivation. Then we learn that CaM kinase II phosphatase is more sensitive to calyculin A relative to okadaic acid, typical of PP-1 activity. This work hints to the fact that CaM kinase II dephosphorylation /deactivation in pancreatic b-cells is controlled by the joint action of phosphatase(okadaic acid sensitive) and PP-2C. (a Mg++- dependent phosphatase)