We determined water budget in the reproducing female common spiny mouse (Acomys cahirinus) offered ad libitum millet seeds and water. We hypothesized that the proportional increase in water input in lactating mice compared with nonreproducing mice would be greater than the proportional increase in energy intake, given that more water would be required for higher heat production and for milk production. Females gave birth to 1 or 2 offspring; results of water budget and energy intake did not differ among litter sizes and were combined. Lactating females increased water input and energy intake by 59.1% and 35.2%, respectively, compared with nonreproducing females, confirming our hypothesis. Milk production (1.04 ml/day) accounted for 29% of the difference in water output between lactating and nonreproducing spiny mice.