Chronic toxicity of a new synthetic corticosteroid, hydrocortisone 17-butyrate 21-propionate (HBP), was investigated in rats of both sexes. HBP was percutaneously given to rats with 0.1%, 0.5% cream and ointment at the daily dose level of 150 mg per 100 g body weight for 6 months. For the comparison, the percutaneous toxicity with 0.12% betamethasone 17-valerate (BV) cream and ointment, and 0.1% hydrocortisone 17-butyrate (HB) cream and ointment at the daily dose level of 150 mg per 100 g body weight were studied. Rats receiving HBP showed the dose-dependent changes such as the suppression of body weight gain and food intake, emaciation, decrease in the number of white blood cells and lymphocytes, total protein, increase in the number of red cells, hematocrit, hemoglobin, blood sugar and total cholesterol, regressive changes in adrenal cortex, lymphatic and hematopoietic tissues and skin, and gastric erosion, which have been well known as toxic effects of synthetic corticosteroids. These findings were comparatively high toxic in male, and almost disappeared in rats elapsed recovery time of month after withdrawal of HBP. The toxicities of HBP, BV and HB were qualitatively same. However, the grade of effects of HBP toxicity was similar to that of HB, weaker than of BV.