The role of mast cells and their main secretory products in the effects of oestradiol on the uterus was investigated. Ovariectomized rats were treated with a single injection of oestradiol (10 micrograms per rat, i.m.) or vehicle together with drugs affecting the activity of mast cells, cromoglycate (10 mg per rat, i.m.), which diminishes the degranulation of mast cells, or compound 48/80 (0.5 mg per rat, i.m.), which enhances this process. Oestradiol or vehicle was also administered with two important secretory products of mast cells, heparin (0.4 mg per rat, i.m.) or histamine (2 mg per rat, i.m.). All drugs were injected simultaneously with oestradiol (first injection) and then every 6 h until the animals were killed. Observations were performed at 24, 36 and 48 h after oestradiol or vehicle injection. The condition of mast cells was determined by the percentage of degranulated mast cells in sections stained with toluidine blue. Oestradiol-induced effects in the uterus were estimated by the mitotic index, proliferating cell nuclear antigen-labelling index, DNA content, volumes of cells, nuclei and nucleoli in the luminal epithelium, glandular epithelium and stroma cells of the endometrium. Cromoglycate treatment resulted in a decrease in both mast cell degranulation and all examined oestradiol effects in the uterus at all periods of observation. Compound 48/80 increased mast cell degranulation and expression of one aspect of oestradiol effects on the volumes of cell compartments. Histamine or heparin led to a marked increase in the cell, nucleus and nucleolus volumes in all uterine structures. However, heparin produced a depression in proliferation, whereas histamine had a weak transient stimulating action on this process. No effects of the protocols were found in the absence of oestradiol treatment. These results suggest that mast cells are involved in the realization of oestrogen action, including the stimulation of cell growth and proliferation in the uterus, and that the effect of mast cells is mediated by both histamine and heparin.