Energy allocation for maternal maintenance and milk production was examined in lactating hispid cotton rats (Sigmodon hispidus) supporting three to seven offspring at 10 degrees or 24 degrees C. Lactating mothers obtained most of their energy from dietary intake (ca. 90%), and the remainder was withdrawn from maternal stores. There was no indication that a central limit to maternal energy assimilation constrained lactational performance. Maternal energy assimilation increased with a larger litter size (a higher production cost) and a decline in ambient temperature (a higher thermoregulatory cost) during lactation, without reaching an apparent limit. Further, there was no evidence of competitive energy allocation, which might occur if maternal energy assimilation were limited. Hence, increases in maternal thermoregulatory expenditure during lactation did not decrease the energy allocation for milk production. Lactating mothers had a capacity to increase milk production. Nonetheless, the milk flow did not fully satisfy the energy requirements of dependent offspring in larger litters or at the lower ambient temperature (growth rates of offspring declined in both cases). Local physiological constraints and behavioral effects appear to limit maternal allocation during lactation. Constraints to allocation may be favored by selection because they reduce maternal risk or reproductive cost.