Water extracts of 10 medicinal plants were checked for protection against obesity in female and male (SLN x C3H/He) F1 obese mice. Each extract was given as drinking water at the concentration of 0.5% between 3 and 32 weeks of age. Among agents examined, only the root extract of tree-peony (Paeonia suffruticosa Andr) retarded growth, especially in males. In males, the treatment with tree-peony resulted in a significant decline in food intake and of the Lee index, an index of obesity, and an increase in glucose tolerance. While no significant difference was observed between the experimental and the control groups in serum free fatty acid levels, the range of the levels was much larger in female and male mice given tree-peony. There was little difference between groups in the weights of heart, liver, kidney, lung, spleen and major endocrine organs in both sexes and in the pattern of oestrous cycles in females. All results indicate that tree-peony can protect against obesity, especially in males, at least partly by a decrease in food intake and an increase in glucose metabolism.