Physiological limits to energy budgets were estimated in Djungarian hamsters (Phodopus sungorus) using food balance and respirometric methods. The summer acclimatized, reproductively inactive hamsters could balance their energy budget at-2° C, assimilating 91.1 kJ·animal-1· day-1 after gradual cold acclimation, whereas non-acclimated hamsters showed negative energy balance assimilating only 54.4 kJ·animal-1·day-1. At the same ambient temperature, multiparous females (although neither pregnant nor lactating at the time) maintained positive energy balance assimilating 81.6 kJ·animal-1·day-1. Hamsters are capable of rapid adjustments of their maximum assimilation rates to meet their current energy demands, but only up to the value of about 3.5xBMR. It is concluded, that the actual energy budgets of small mammals keep, all the time, fairly near the upper physiological limit, with body reserves ready to buffer short-term oscillations.