The rate of progression of spermatogenesis was studied in immature Djungarian and Chinese hamsters and Wistar rats by scoring the most advanced cell types present at various ages after birth. From 15 days of age onward, the most advanced cell types in the Djungarian hamsters were formed at a rate compatible with the duration of the spermatogenic cycle in adult animals, i.e., 7.9 days. However, in Djungarian hamsters up to 15 days of age, the rate of spermatogenic development was accelerated. The estimated duration of the spermatogenic cycle ranged between 5.0 and 5.3 days. In the rats, spermatogenesis also was accelerated during the first 15 days of life, with an estimated duration of the seminiferous cycle of 4.5-5.3 days. From 15 days of age onward, the rate of progression was strongly reduced, being compatible with the adult value of 12.8 days. In the Chinese hamsters, a similar change in the rate of spermatogenesis occurred at 25 days. Before this age, spermatogenesis proceeded with an estimated duration of the cycle of 8.8-9.2 days. From 25 days onward, spermatogenesis advanced much more slowly, at a rate compatible with the adult value of 17.0 days. Despite the strong reduction in the rate of spermatogenic progression in the three species, the cellular associations in the stages of spermatogenesis were not affected. In the three species, the clear reduction in the rate of spermatogenic progression correlated with the process of testicular descent, with the appearance of pachytene spermatocytes associated with preleptotene spermatocytes, and with the onset of tubular lumen formation.(ABSTRACT TRUNCATED AT 250 WORDS