The effects of dithiocarb and (+)-catechin on the microsomal mixed-function oxidase system of rat liver were investigated after a single dose as well as after repeated treatment for 7 and for 28 days. Dithiocarb (200 mg/kg p.o.) significantly reduced the microsomal cytochrome P-450 content, aniline hydroxylase and aminopyrine demethylase activities; maximum decrease was observed at 4 hrs, return to normal values after 24 hrs. (+)-catechin (200 mg/kg p.o.) had no effects in this respect. Both agents did not affect microsomal enzyme activities when applied orally for 7 days. After 28 days treatment only dithiocarb (50 - 100 - 200 mg/kg p.o.) exerted a dose-dependent depression of the aniline hydroxylase activity. In vitro experiments confirmed the in vivo observations: a concentration-dependent inhibition of the aniline hydroxylation and aminopyrine demethylation were seen from the addition of dithiocarb, the I50-values were 5.4 X 10(-6) M and 9.6 X 10(-5) M, respectively. (+)-catechin showed no inhibitory activity on both enzyme activities in vitro. Both dithiocarb and (+)-catechin depressed the activity of the NADPH-cytochrome C-reductase only in the 10(-4) M concentration range, these effects should be therefore evaluated as non-physiological without relevance in vivo.