The thermogenic activity of ephedrine, ethinyloestradiol and triiodothyronine were compared by incorporating the drugs into the diets fed to groups of adult rats and mice. The animals used were normal lean mice, hypothalamic obese mice, dietary-induced obese mice and rats, and genetically obese mice and rats. Food intake and body weight were monitored throughout and oxygen consumption measurements were made. Finally, the animals were killed and their carcasses analysed. Generally, ethinyloestradiol reduced body weight by causing anorexia. Triiodothyronine increased oxygen consumption but also increased food intake such that in most cases body weight did not change; it killed the genetically obese animals, but reduced fat in the hypothalamic and dietary induced obese animals. The animals treated with ephedrine lost body weight and body fat without increasing food intake, and had elevated oxygen consumptions. This experiment not only demonstrated marked differences between the various laboratory animal models of obesity, but also that ephedrine is a potential slimming agent. It is relatively safe and has been used by us to obtain successful breeding using the genetically obese male animals which are normally sterile.