Time-courses of changes in the activities of liver and kidney glucose-6-phosphatase [EC 126.96.36.199] and hepatic tryptophan pyrrolase [EC 188.8.131.52; TPO] in rats pre-fed high-protein diets for 5 days and then shifted to zero-protein diets were studied. Liver glucose-6-phosphatase activity decreased 1 day after the dietary shift but then increased and remained significantly higher than the 0 day value for the next 2 days. Changes in liver glycogen were found to be intimately and inversely related to liver glucose-6-phosphatase activity. Changes in kidney glucose-6-phosphatase activity paralleled the pattern of changes observed in liver activity. An initial decrease in TPO activity was followed by increased enzyme activity up to the 3rd day of the dietary shift. Later there was a rapid fall in tryptophan pyrrolase activity. Changes observed in these specific enzyme proteins differed from those observed in total tissue proteins. Alterations in the activities of these enzymes and changes in other parameters are compared with those observed earlier with the reverse type of dietary shift.