17β oestradiol (E 2 ) or ethinyl oestradiol (EE) was administered daily by gavage for 52 weeks to Beagle dogs at the dose levels of 5 mg/kg/day E 2 and 0.5 mg/kg/day EE or 2 mg/kg/day which was then decreased to 1.5 and 1 mg/kg/d EE. The dose levels in Cynomolgus monkeys were 5 mg/kg/day E 2 and 2 mg/kg/day EE. The effect of hormonal treatment on clinical signs, food consumption, body weight, haematological and blood biochemical parameters were evaluated. Treatment with both oestrogens induced significant mortality in dogs only. Brownish/blackish pigments on the abdomen, small testes and mammary gland swelling were found in both species. Severe alopecia was noted only in dogs. Lower food consumption was observed in dogs and decreased body weight was found in both dogs and monkeys. The effect on body weight was more marked in dogs than in monkeys and more prominent in males than in females. Moderate to severe thrombocytopenia was noted in a cyclical manner in dogs, the effect being more prominent in animals treated with EE than those treated with E 2 . This thrombocytopenia was one of the major factors contributing to death among the treated dogs. No thrombocytopenia was found in monkeys. In both dogs and monkeys, normocytic nonregenerative anaemia was observed. This was associated with neutrophil leucocytosis in dogs and with prolonged activated partial thromboplastin time in monkeys. Lower inorganic phosphorus was noted in both species. This was associated with higher cholesterol level in dogs and lower alkaline phosphatase activity and higher triglyceride level in the monkeys. These results demonstrated that the dog is more sensitive to oestrogen treatment than the monkey. Considering the known effects in humans, the latter seems to be a more suitable model for toxicological testing of such compounds.