Protein phosphatase 1 and protein phosphatase 2A contain potential phosphorylation sites for cyclin-dependent kinases. In the present study we found that rabbit skeletal muscle protein phosphatase 1, as well as recombinant protein phosphatase 1 alpha and protein phosphatase 1 gamma 1, but not protein phosphatase 2A, was phosphorylated and inhibited by cdc2/cyclin A and cdc2/cyclin B. Phosphopeptide mapping and phospho amino acid analysis suggested that the phosphorylation site was located at a C-terminal threonine. Neither cdc2/cyclin A nor cdc2/cyclin B phosphorylated an active form of protein phosphatase 1 alpha in which Thr-320 had been mutated to alanine, indicating that the phosphorylation occurred at this threonine residue. Furthermore, protein phosphatase 1, but not protein phosphatase 2A, activity was found to change during the cell cycle of human MG-63 osteosarcoma cells. The observed oscillations in protein phosphatase 1 activity during the cell cycle may be due, at least in part, to phosphorylation of protein phosphatase 1 by cyclin-dependent kinases. Together, the results suggest a mechanism for direct regulation of protein phosphatase 1 activity.