Three groups of 24-day-old male Wistar rats were fed (a) a control diet containing 210 g/kg protein ad libitum (control) (b) a low protein diet consisting of the stock diet diluted with starch to contain 70 g/kg protein ad libitum (protein deficient) or (c) an amount of stock diet containing the same amount of protein as eaten by (b) but without starch (energy deficient). The low protein diet did not affect the ability of the animals to metabolize biphenyl and excrete it as 4-hydroxybiphenyl, or reduce the total activity of 4-hydroxylase per liver, whereas this was reduced by restricted amounts of the high protein diet. The low protein diet resulted in raised levels of corticosteroids in the plasma. It is concluded that the raised specific activity, and unchanged total amount of biphenyl 4-hydroxylase per liver of the low protein animals is an adaptation to the diet mediated by corticosteroids as exogenous corticosteroids were also found to increase the activity of this enzyme in normal animals.