ABSTRACT Long-term ovariectomized rabbits were given injections of progesterone and prolactin in an alternating sequence (progesterone for 5 days, prolactin for 4 days and progesterone for 5 days) to test the hypothesis that each of these hormones acts to increase the other's receptor, resulting in a positive feedback process whereby prolactin augments the progesterone-dependent increase in the mRNA for the uterine protein, uteroglobin. The results of this study support the hypothesis, in that the experimental rabbits produced exceptionally large concentrations of uteroglobin.