['To investigate the effects of paternal age on fertility and progeny outcome using the Brown Norway rat model.', 'Controlled prospective study.', 'McIntyre Animal Centre, McGill University, Montreal, Quebec, Canada.', 'Brown Norway male rats of increasing age were mated to young Sprague-Dawley females.', 'Pregnancy outcome was assessed by counting the numbers of corpora lutea, resorptions, and live fetuses on day 20 of gestation. To evaluate progeny outcome, pups were examined for external malformations and weighed daily for 2 months.', 'There were no significant changes in the numbers of resorptions, offspring, or in the incidence of external malformations. However, there was an increase in preimplantation loss (corpora lutea minus implantation sites) in litters fathered by older males. Furthermore, a significant decrease in the average fetal weight was found with increasing paternal age. A significant increase in neonatal deaths for progeny fathered by older males also was found.'] Does paternal age affect fertility and progeny outcome in the Brown Norway rat?