Abstract 1. 1. Biochemical and functional effects of a single dose of fenfluramine were studied in maturing mice aged 1 day or 1, 2 or 6 weeks postpartum. 2. 2. 5-HT and 5-HIAA stores in hemispheres and brain stem were significantly reduced at all ages 1 day after the drug injection. 3. 3. In contrast to adult animals which showed continuing reductions in 5-HT and 5-HIAA 4 weeks after the fenfluramine injection, animals at all younger ages returned to normal by 1 week after the injection. 4. 4. Body weight was reduced 2–4 weeks after fenfluramine injection in 1 week old animals; however 2 week animals showed only a small transient reduction during the first week following fenfluramine. 5. 5. No significant effects were found on body temperature or temperature preference. 6. 6. The results are discussed in relation to the effects of various amine depleting agents.