The effects of neonatal exposure to different doses of diethylstilbestrol (DES) on the reproductive functions of male rats at adulthood were evaluated. Sprague-Dawley rats (5–8/group) received sc injections of 25 µl olive oil containing DES (Sigma Chemical Co., St. Louis, MO) at a dose of 10 µg, 1 µg, 100 ng, 10 ng, or 1 ng per rat on alternate days from Postnatal Days 2–12. Control animals received olive oil only. All animals were allowed to develop until 83–91 days of age; however, when they were 70 to 80 days old, four male rats each from the 10 µg, 1 µg, 100 ng, and control groups were cohabited with untreated 60- to 70-day-old females (1:1) for 12 days. At the end of cohabitation, both mated and unmated male rats were weighed, and blood and tissue samples were collected and processed. Results revealed that although sperm motility patterns and sperm morphology were adversely affected in the 10-µg group, other reproductive parameters, including 1) daily sperm production (DSP)/testis; 2) absolute and relative weights of the testis, epididymis, and seminal vesicle; and 3) sperm numbers in both regions of the epididymis declined significantly in a dose-dependent manner in the 10- and 1-µg groups. Conversely, in the <1-µg groups, none of these parameters (except DSP/testis and weight of the epididymis in the 100-ng group, and sperm numbers in the epididymis of the 100- and 10-ng groups) was different from controls. Generally, plasma testosterone levels decreased in the 10- and 1-µg groups, FSH level increased in the 10-µg group, and prolactin and LH levels were unaltered. In the fertility study, although each male in the 1-µg, 100-ng, and control groups produced a copulatory plug and impregnated a female, none could do so in the 10-µg group. The mean number of pups per litter was reduced to eight in the 1-µg group, in contrast to 15 each in the 100-ng and control groups. In conclusion, exposure of neonatal male rats to DES altered sperm motility patterns, sperm fertility (as evident from the reduced number of pups in the 1-µg group), and sexual behavior (as evident from the absence of copulatory plugs in the 10-µg group) and reduced weights of reproductive organs, DSP/testis, and sperm numbers in the epididymis. Whether these alterations/reductions persist in older rats (6–8 mo of age) is under investigation.