Captopril is the first oral inhibitor of angiotensin-converting enzyme, which has a strong effect on patients with hypertonic states. The influence of the preparation was studied on liver drug-metabolizing enzymic systems of rats in two ways of oral administration-single and a 10-day treatment with a dose of 75 mg/kg. The effect on the levels of microsomal cytochrome B5 and P450 as well as on that of hem was investigated. The activities of some enzymes from the first and second phases of biotransformation were determined. There no changes in the examined parameters 1 h after single oral treatment with 75 mg/kg of captopril. Induction of some of the examined enzymes was observed after a 10-day treatment with 75 mg/kg r. o. of captopril once daily: there was statistically significant elevation of the level of cytochrome P450 as well as of the activity of ethylmorphindelethylase and HA-DPH-cytochrome-C-reductase. The results show that there is probably induction at the first phase of phenobarbital type of drug metabolism.