Distribution of progesterone receptor (PR) was studied immunohistochemically in rabbit ovary and uterus using a monoclonal anti-receptor antibody. At the subcellular level, PR was located in cell nuclei of prepubertal rabbits, which were either non-treated, primed with estrogen or made pseudopregnant. At the tissue level, germinal epithelium, the external theca cell layer and granulosa cell layer were PR-positive. The internal theca cell layer had no PR immunoreactivity. The corpora lutea of pseudopregnant rabbits contained small amounts of PR in some of the animals. During pseudopregnancy, the distribution and staining intensity of PR-positive cells in the ovary was essentially similar to that in non-treated rabbits except for weak PR immunoreactivity in the internal theca cell layer. This differed significantly from the uterus, in which pseudopregnancy caused a marked decrease in PR immunoreactivity. This implies that receptor downregulation by endogenous progesterone is not the same in different organs and cell types. Immunohistochemical techniques give valuable information as to the steroid hormone target cell types and their distribution which are not available by conventional steroid-binding assays.