Content and functional activity of rat liver microsomal haemoproteins were studied after injection of heliotrin (HT). Heliotrin, at a dose of 30 mg/100 g, decreased distinctly cytochrome P-450 concentration in microsomes and increased the rate of inactivation of the enzyme reduced form. Decrease in the NADPH and NADH-dependent flavoprotein activities, decrease in O-dealkilating and p-hydroxylating activities and a distinct increase in NADPH- and ascorbate-dependent peroxidation of membrane lipids were found in rat liver microsomes treated with HT. As shown by polarographic analyses HT influenced the increase in the rate of NADPH oxidation similar to the effects caused by typical substrate of hydroxylation dimethylaniline. When the binding spectra were estimated, HT proved to have high affinity to cytochrome P-450. The data obtained suggest that HT appears to interact with the microsomal monooxigenase liver tissue system.