In the group of 107 patients poisoned by carbon monoxide (18 patients), ethanol (10), barbiturates (18), glutehimide (10), tranquilizers (19), organic solvents (10),salicylates (3), organochlorines (8), and sulfonamides (5)--the activities of 8 serum enzymes were determined for 6 consecutive days of treatment, the enzymes being as follows: aminotransferases, cholinesterase, alkaline phosphatase, lactate, alpha-hydroxybutyrate, glutamate, and sorbitol dehydrogenase. The antipyrine half-life was also assayed. It has been shown that the poisonings by particular groups of poisons do not bring about characteristic changes in the activity of enzymes that might be of any diagnostic value. The intensity of changes was connected withe depth and duration of toxic coma. Most frequently an increase ensued in the activity of AspAt and AlAt in the third 24-hrs period, and an increase in the activity of SDH in the first 24-hrs period. In the group under examination there were 26 drug abusers in whom a shortening of the antipyrine half-life was discovered. They were less responsive to toxic doses of drugs, and the enzymatic changes in them were less distinct. No changes in the activity of tested enzymes, which are characteristic of toxicomania, were found.