The distribution of proliferating cell nuclear antigen (PCNA) in specific somatic and germ cells of the adult mouse ovary and testis was assessed using immunocytochemical staining and immunoblot analysis and was correlated with cellular proliferation and differentiation. In the adult ovary, immunocytochemical staining for PCNA within follicular cells varied depending on the stage of follicular growth. Since PCNA staining has proven to be a useful indicator of cells involved in DNA synthesis and repair, the pattern of PCNA staining in the ovary was compared to previous studies which used tritiated thymidine labeling as a marker for DNA synthesis. In the testis, PCNA was detected in the mitotically proliferating spermatogonia, but not in spermatocytes which had just entered meiosis. PCNA staining was again observed in spermatogenic cells in later stages of meiotic prophase, in particular zygotene and pachytene spermatocytes. As these cells are undergoing meiotic recombination, the presence of PCNA in these meiotic prophase cells could reflect a second function of PCNA, that of DNA excision repair.