Abstract 1. 1. Prolonged treatment of guinea-pigs and rabbits with ethchlorovynol, methylparafynol, or ethinamate resulted in an enhancement of the activity of δ-aminolevulinic acid (ALA) synthetase as judged by the increase in the amounts of hepatic ALA, although to different extents. 2. 2. A similar but less pronounced effect was noticed after acute treatment with the drugs. 3. 3. Evidence is produced to support the thesis that the observed enzyme-induction was mainly due to de novo synthesis of enzyme protein and possibly to increased quantities of enzyme substrate. 4. 4. The drugs were not specific in this respect, since they were also capable of enhancing the activity of other enzymes such as glutamic dehydrogenase and glutamic-oxaloacetic transaminase.