Hormones have the capacity to alter the physiology and behaviour of animals. This has important implications for an individual's life history, especially considering the central role of sex hormones in reproductive processes. Studies of the bank vole (Myodes glareolus) have revealed the effects of testosterone (T) on sexual selection. Using offspring of wild-caught individuals, we artificially selected for high and low T. We measured the effect of testosterone selection on life history traits, including basal metabolic rate (BMR), reproductive success, litter measures as well as sperm characters. Testosterone had a different effect on metabolism in males compared to females. An antagonistic sex difference was also found in the way T affected reproductive success: high T males and low T females were the most successful. Furthermore, there was evidence of an effect of T on the siring ability of sperm based on sperm characteristics. Overall, we found that testosterone has a potent effect on reproduction in this wild species. The role of testosterone in physiological processes, reproductive behaviours and the evolution of this common European mammal is discussed.