The aim of this study was to examine the effect of growth hormone on the uterine response to oestradiol. Ovariectomized rats were treated with a single injection of oestradiol dipropionate (10 μg per rat, i.m.), together with a single injection of human growth hormone (0.04 mg per rat, i.m.). The effects of oestradiol in the uterus were determined by measuring the following: mitotic index; the percentage of cells positive for the proliferating cell nuclear antigen (using immunocytochemistry); the DNA content (using Feulgen's method); and the volume of cells, nuclei and nucleoli (using morphometry) in the luminal epithelium, glandular epithelium and stroma of the endometrium 24, 36 and 48 h after injection with oestradiol. All values for all structures were decreased after oestradiol and growth hormone treatment compared with values of rats treated with oestradiol only. However, parameters that reflected proliferative processes (mitotic indices, the number of cells positive for proliferating cell nuclear antigen, and DNA content) were much more reduced than those indicating cell growth (volumes of cells, nuclei and nucleoli). In the absence of oestradiol, administration of growth hormone did not influence the uterine parameters tested. The effect of growth hormone is likely to be brought about by changing the mechanism of oestrogen action on the uterus.