The inhibition of stress induced increase of liver enzyme activity by progesterone occurs in rats of both sexes.

Adult rats of both sexes were injected an aqueous suspension of progesterone in a dose of 12.5 mg i.p. + 12.5 mg i.m. and 50% of the animals were subjected to immobilization stress for 150 minutes either immediately after the hormone administration or at various time intervals after that. Non injected rats served as controls. After decapitation the plasma corticosterone level and the activity of tyrosine aminotransferase and tryptophan pyrrolase in the liver was determined. It was found that in non immobilized animals all three parameters investigated were increased to a minor degree 3 hours after progesterone injection. In stressed rats the stimulatory effect of stress induced hypercorticosteronaemia on the activity of both enzymes was decreased at 14 and/or 20 hours after progesterone administration, no sex differences being observed. The potential usefulness of progesterone as inhibitor of exaggerated stress is underlined.