The effect of cyclosporine A (CsA), the immunosuppressant used in transplantation and also in the treatment of some autoimmune diseases, on the microsomal mixed function oxidase (MFO) systems in rat kidney and liver was studied. Since CsA given intragastrically (50 mg/kg/day) for three consecutive days decreased body, liver and kidney weights, in rats, the results were compared not only with the control untreated animals but also with the group of fully starved rats. In the liver the cytochrome P-450 level and aniline-hydroxylase activity were slightly higher than in the control rats but not as high as in the fully starved animals. This suggests that in the liver the effect might be the result of the antagonism between the CsA action and partial starvation of the CsA-treated animals. On the other hand, in the kidney the cytochrome P-450 level was as high as in the fully starved animals and the aniline-hydroxylase activity was significantly increased as compared to both the control and fully starved animals. Thus, in the kidney the microsomal MFO system seems to be induced after short-term CsA treatment. The activities of aminopyrine-N-demethylase and the levels of cytochrome bs did not change significantly after CsA treatment in both organs.