1. DL-Fenfluramine, a 5-hydroxytryptamine (5-HT) agonist, increases the metabolic rate of the adult domestic fowl and also stimulates heat loss mechanisms. Since thermogenic mechanisms vary qualitatively and quantitatively with age, this series of experiments was designed to measure the effects of fenfluramine on immature, growing, 'broiler' chickens. 2. The work was also designed to distinguish any direct effect of fenfluramine on respiratory quotient (RQ) from the indirect effects of reduced food intake. 3. Female commercial broilers were used (5-7 weeks old; 1.1-1.8 kg), at an ambient temperature of 25 degrees C and on a 14 hr light: 10 hr dark cycle. DL-Fenfluramine was injected intramuscularly at 80 mumol/kg body weight (20 mg/kg), in 0.5 ml of 9 g/l saline per kg bodyweight. 4. When the effects on heat production (H) of differences in food intake were eliminated by fasting, fenfluramine increased H by up to 20% within about 20 min. 5. RQ decreased four times more rapidly after fenfluramine injection than after the control treatment whether the birds were fed or fasted, reaching 0.70 in about 3 hr. 6. Wing-drooping and polypnea occurred within 10 min of injection. Maximal response was in 30 min to 1 hr for polypnea and 30 min to 2 hr for wing-drooping, depending on experimental procedure. Both responses ceased in 5-7 hr. 7. Physical activity was greatly reduced by fenfluramine, particularly in the first 4 hr. This effect occurred whether food was available or not.(ABSTRACT TRUNCATED AT 250 WORDS)