Slices of rat corpora lutea(CL) incubated with. prostaglandin F_2α(PGF_2α) in Krebs-Hensenleit (K-H) Ringer solution showed a decrease in Na⁺-K⁺-ATPase activity after 60 min of incubation. However, PGF_2α in vitro did not alter Na⁺-K⁺-ATPase activity of isolated luteal membrane fractions. Following PGF_2α induced in vivo luteal regression, reduction of Vmax an elevation of the activation energy above transition temperature of the lipid phase of the membrane occurred without changes of Km, optimum pH and transition temperature. These results suggest that reduction of Na⁺-K⁺-ATPase activity after PGF_2α treatment may be due to the reduction of the number of enzyme molecules or to masking of the active site of the enzyme without any change in enzyme characteristics. In addition, a change in membrane bound enzyme activity may be an early step in PGF_2α induced luleolysis.