The ultrastructural changes in hepatocytes of steers were studied in animals implanted with 140 mg trenbolone acetate, alone or in combination with 20 mg oestradiol. The mean volume of the nucleus in the hepatocytes of the animals implanted with trenbolone acetate alone was significantly greater than that in control animals (P less than 0.001) and in animals implanted with the combined preparation (P less than 0.05). It is suggested that the increase indicates an androgen-mediated response in the nuclei which is not seen when oestradiol is included in the implant. Similarly, the peroxisome volume density was elevated significantly above that in the controls by trenbolone acetate (P less than 0.001); although the inclusion of oestradiol in the implant significantly reduced this level (P less than 0.001) it was still greater than the control values (P less than 0.05). No mitochondrial abnormalities were observed.