In species where there is postcopulatory competition between males in the form of sperm competition, theory predicts that males should show prudent sperm allocation. Specifically, males should adjust the number of sperm that they inseminate in response to variations in the risk and intensity of sperm competition. Some experimental evidence for nonmammalian species indicates that ejaculate expenditure can be dynamic and adjusted in accordance with sperm competition theory. However, most studies have not used within-subject experimental designs to demonstrate ejaculate adjustments by individual males, and there have been no unequivocal demonstrations of prudent sperm allocation in mammals. In the present study, copulatory ejaculates were collected from 12 male rats that each mated under two experimental conditions: alone in a cage with a female (monandrous) and (accompanied by a ‘rival’ male who observed the copulation from behind a perforated plastic screen (polyandrous). As predicted by sperm competition theory, males ejaculated significantly more sperm when copulating in the presence of a rival than when copulating alone. Furthermore, the magnitudes of these ejaculate adjustments were highly predictable and were relatively consistent across individual males.