Organ blood flow, as estimated by the radioactive labelled microsphere technique, was studied in rats before and after intracaval administration of the calcium antagonist nifedipine. Intra-uterine and arterial blood pressures were registered simultaneously. Two groups of animals were investigated, one non-treated and one treated with oestradiol. After administration of nifedipine, an increase in the blood flow to the brain, the heart and the kidneys was seen in both groups of animals. However, in the uterus, blood flow was markedly increased in non-treated, but not so in oestradiol-treated animals. The arterial blood pressure and intra-uterine pressure were almost unaffected. The results suggest that nifedipine can markedly increase blood flow to the uterus of non-oestrogen-treated rats with only minor effects on intra-uterine and arterial blood pressures.