Testicular degeneration was observed in exploratory toxicity studies in Wistar rats treated with several mGluR5 negative allosteric modulators. To determine if these testis effects were influenced by animal age, these compounds were administered to male Wistar rats of different ages (8, 10, and 12 weeks old) for 2 weeks followed by evaluation of male reproductive organ weights, testis histopathology, and inhibin B levels. Overall, seminiferous tubule degeneration was observed in 2/15, 5/15, and 0/15 compound treated rats from the 8, 10, and 12 week old cohorts and inhibin B was decreased in 8 and 10 week old animals, but not in 12 week old rats, suggesting that there is an age-related component to this testis toxicity. The gene expression profiles of drug transporters in the testis of rats aged PND 38 through PND 91 were very similar, indicating that immaturity of these transporters is an unlikely factor contributing to the age-related toxicity.