Nuclear type 1 protein phosphatase (PP1) activity in primary culture of EGF‐stimulated hepatocytes was significantly and transiently increased at the G1/S transition, being about 2.5‐fold, while that in non‐stimulated hepatocytes showed almost no change. On the other hand, non‐nuclear PP1 activity was gradually increased until the G1/S transition, but the activity showed no difference between EGF‐stimulated and non‐stimulated hepatocytes. Under growth‐inhibited conditions in the presence of TGFβ1, the increase in nuclear PP1 activity was completely suppressed, whereas non‐nuclear PP1 activity was little affected. Such close correlation between nuclear PP1 activity and growth factor‐induced positive or negative growth signaling strongly suggests an involvement of PP1 in progression from G1 to S phase of hepatocytes. On Western immunoblotting using antisera for PP1α, PP1γ1, and PP1δ, no isoform showed any change in amount under these conditions. Mechanism(s) of growth‐associated alterations in nuclear PP1 activity is discussed.