Molecular forms of hepatic pyruvate kinase (PK) were separated by fractionating on DEAE-cellulose. 120-h food deprivation of rats entails a progressive decline in L-PK activity, but not the activity of M-type enzyme of the minor fraction. The rate of L-PK degradation depends on the fasting duration. A rapid inactivation phase is followed by a slower one with the speed constants 0.023 and 0.0065 h-1, respectively. To control the L-PK degradation rates in fasting diets, protein modification by phosphorylation can be employed.