The proliferation and differentiation of mast cells and Leydig cells were studied in adult sham operated or hypophysectomized rats after the administration of ethylene dimethane sulphonate (EDS) and in prepubertal rats after neonatal treatment with a gonadotropin-releasing hormone (GnRH) antagonist (Organon 30276; Oss, The Netherlands). After treatment with EDS, two proliferative waves were found. On day 3, several interstitial cell types proliferated, whereas mitotic cells corresponded to differentiating Leydig cells and mast cells around day 20. Differentiating Leydig cells showed a higher mitotic index than that of differentiating mast cells. Hypophysectomized animals showed high mitotic activity 3 days after treatment, but 21 days after treatment differentiating Leydig cells were absent and proliferative activity was reduced. The number of mast cells increased from day 15 to day 30 in EDS-treated rats and from day 15 to day 50 in hypophysectomized, EDS-treated rats. GnRH antagonist-treated rats showed poorly differentiated Leydig cells and abundant mitotic figures on day 23. Proliferation and differentiation of Leydig cells occurred concomitantly with the proliferation and differentiation of mast cells between 23 and 30 days of age. These results suggest that Leydig cells and mast cells in the rat testis share some common regulatory factors.