Male rats were exposed to 17 alpha-ethinylestradiol (EE2) in their drinking water either at concentrations of 1-1000 ng/mL (1 ppb to 1 ppm) for 3 weeks immediately after weaning from post-natal day 22 (PND 22) to PND 43 or at a concentration of 10 ng/mL from PND 5 onwards. Although given temporarily between PND 22 and PND 43, the 0.1 and 1 ppm doses of EE2 were found to significantly affect the ponderal growth of the animals. No male was found to be sterile neither in both exposed groups nor in control groups. Nevertheless, the males exposed to 10 ng/mL EE2 in drinking water from PND 5 onwards conceived a significantly higher proportion (25%) of small litters (one to five pups) than control males (0-3%). When exposed to 1 mu g/mL EE2 but only from PND 22 to PND 43, the males produced an intermediate proportion (15%) of small litters in their progeny but this might be consecutive to the observed retardation in weight gain. (C) 2007 Elsevier Inc. All rights reserved.