The present study investigated the dose-response relationship in testicular toxicology of pubescent (7-) and adult (17-week-old) Sprague-Dawley rats whose dams had been injected (i.g.) with 25 pg, 2.5 ng, 250 ng, or 7.5 μg of 3,3′,4,4′,5-pentachlorobiphenyl (PCB126)/kg or the vehicle on days 13 to 19 post-conception. Rat at 7 and 17 weeks of age of the 7.5 μg group and at 7 weeks of age of the 250 ng group showed an increase in the percentage of seminiferous tubules at Stages VII-VIII. At 7 and 17 weeks of age, rats of the 7.5 μg group showed a decrease in preleptotene spermatocytes with spermatids at all Stages, while animals of the 250 ng group also showed a decrease in preleptotene spermatocytes, but with round spermatids increasing at Stages VI-VII and elongated spermatids decreasing at Stage VIII. At 7 weeks of age, rats of the 2.5 ng group showed an increase in round spermatids at Stages VI-VII. The formation of spermatogenic cells in rats of the 25 pg group was similar to that seen in the vehicle group. The number of Sertoli cells and cauda epididymal sperms in rats of the PCB126 groups were similar to those of the vehicle group. Prenatal PCB126 exposure induced dose-related defective spermatogenesis. A high dose of PCB126 affected the development of spermatogonia and spermatids in puberty and adulthood, while a low dose affected the conversion of spermatids at puberty, although this was recovered in adulthood. Because the serum testosterone levels were similar in the PCB126 and vehicle group rats in puberty and adulthood, a direct endocrine cause for the observed effects was unlikely.