During summer and early autumn, reindeer and caribou (Rangifer tarandus) must exceed daily maintenance requirements for protein and energy to replenish body reserves for winter survival and reproduction. We estimated the maintenance requirements and the incremental costs of mass gain of captive female Siberian reindeer (R. t. tarandus) during summer and autumn by a mass balance approach. Intake (88 versus 56 g kg^−0.75 day⁻¹) and digestibility (89% versus 81%) of dry matter were greater in summer than in autumn on the same diet. Body mass increased over the summer and remained stable into autumn. At zero mass gain, reindeer required 911 kJ kg^−0.75 day⁻¹ digestible energy and 1.12 g kg^−0.75 day⁻¹ digestible nitrogen (N). Requirements of N were affected by endogenous losses of N in the feces that were 72–82% of total fecal N. During summer, reindeer would need to consume forage containing more than 9.5 ± 0.4 (SE) kJ g⁻¹ digestible energy and 1.17 ± 0.4 (SE) % digestible N to gain body mass at intakes of 98 g kg^−0.75 day⁻¹. Reindeer could use a mix of graminoids and browse to meet energy demands and gain fat (6–18 kJ g⁻¹) but may not be able to maintain or gain body protein in late summer as forages senesce and N contents decline below 1% of dry matter.