T-kininogen levels in plasma and liver microsomes were measured by radioimmunoassay in female and male rats of various ages. High levels of T-kininogen were found in plasma and liver of 1-day to 1-week old male and female rats and their mothers. The levels in newborns gradually decreased along with their development. In mature male rats the levels were as low as 1/5-1/2 of those in mature female rats. Treatment with estradiol increased the plasma and the liver levels of T-kininogen significantly in both sexes, but testosterone decreased the level in female rats and had no effect in male rats. These results suggest that sex hormones may regulate the physiological level of T-kininogen in rats.