The size of peritubular Leydig cells surrounding tubules in different stages of the spermatogenic cycle was determined in 43- and 47-day-old male rats. A stage-dependent variation in the size of peritubular Leydig cells was not present in 43-day-old rats, but by 47 days those Leydig cells closely adjacent to tubules at stages VII-VIII were larger than others. At 43 days of age spermatogenesis had developed up to step 18 spermatids in late stage VI tubules. At 47 days of age the first mature sperm had just been released from the seminiferous epithelium, and consequently the first wave of the spermatogenic cycle was completed. Tubules at stages VII-VIII therefore acquire the ability to influence surrounding Leydig cells when they contain step 19 spermatids. It remains to be shown whether this maturation step is due to inherent maturation of the Sertoli cells or if step 19 spermatids specifically modulate Sertoli cell function.