- We determined metabolizable energy intake (MEI), body energy change and milk production in primiparous, lactating Sundevall's jirds (Meriones crassus) when nursing litters of two to six pups. We hypothesized that milk production was a constraint and that pups in small litters would have a faster growth rate than pups in large litters. We also hypothesized that females with small litters would offset the costs predominantly with metabolizable energy intake while females with large litters would be forced to mobilize more body energy. There was a negative relationship between growth rate of a pup and litter size which would imply that milk production was the limiting factor in pup growth, which supports our first hypothesis. Although there was an increase in maternal milk production with an increase in litter size, there was a decrease in milk intake per pup with litter size. There was a significant correlation between loss in body energy and litter size, that is, an increase in litter size was associated with more maternal body energy loss, which supports our second hypothesis. MEI in lactating females was on the average 35% above that of non-reproducing females and, together with litter size explained 74% of the variance in the growth rate of a pup. Each of these independent variables contributed about equally to the variance.