SUMMARY: Uterine luminal fluid accumulation (ULFA) was induced in ovariectomized rats by twice daily treatment for 3 days with 0·5 μg oestradiol-17β. At the end of this treatment, rats were given a single subcutaneous injection of 0·2 ml 0·9% saline containing 0, 0·1, 1, 10, or 100 guinea-pig units (GPU) of relaxin, and ULFA was determined 15–17 h thereafter. It was reduced in animals receiving ≥ 1 GPU relaxin. This reduction in ULFA commenced 12 h after treatment of rats with 10 GPU relaxin, and was completed by 24 h. Ligation of the cervical end of the uterus prevented the loss of fluid in response to a single injection of relaxin, indicating the cervix as the route of fluid loss. Uterine luminal fluid accumulation was reduced to a greater extent by treatment of rats with 1 mg progesterone than by 10 GPU relaxin. Given concomitantly with oestrogen, relaxin had no effect on ULFA in horns which were ligated at the cervical end, indicating that relaxin did not inhibit luminal fluid formation. It is concluded from these results that the effects of progesterone on ULFA are not mediated by relaxin. The results, however, do not allow rejection of the hypothesis that the effect of prolactin on ULFA is mediated by relaxin.