During a systemic tolerance study in rats with compound (5-methoxy-2-thienyl)-thioacetic acid sodium salt (MTTA), which is known to decrease the lipolytic process and platelet aggregation, effects after high doses were observed which are similar to the well-known findings after corticosteroid administration. The oral application of MTTA in a dose range from 10 to 1000 mg/kg over a period of 4 weeks resulted in a decrease in total lipids and free fatty acids from 100 mg/kg onwards and an increase in total glycerin and triglycerides after 1000 mg/kg, supplemented with an increased excretion of keto-bodies in urine at all dose levels. The hematological examinations revealed a reduction of blood eosinophils and lymphocytes in males at doses from 10 mg/kg, whereas the eosinophilic granulocytopoiesis was decreased only at the 1000 mg/kg level in both sexes. Determinations of organ weights showed a decrease in thymus weight from 300 mg/kg onwards with an involution at the highest dose levels, being confirmed by histological examination, whereas the adrenal glands' weights were decreased only at the dose level of 1000 mg/kg. Since the endogenous corticosterone level remained unaltered at all doses, it is suggested that these corticosteroid-like effects are directly attributable to MTTA.