The aim of this study was to obtain ecologically relevant data on the rates and efficiencies of energy allocation to reproduction and to determine how laboratory data compare with data obtained under 'seminatural' conditions. Using the tritiated water (HTO) method we measured average daily metabolic rates (ADMRs) in Phodopus sungorus reproducing females over the whole period of pregnancy and lactation and in nonreproducing ones, under both laboratory and 'seminatural' conditions.Taking the difference in total energy expenditure between reproducing and nonreproducing females as the additional energy requirement for reproduction, it was found that the absolute energy expenditure for reproduction was independent of the experimental conditions used. For the production and nursing of one litter, Djungarian hamsters metabolized an additional 20.9 MJ kg in the laboratory, i.e. a 37% increase over normal maintenance expenditure, and 22.6 MJ kg under seminatural conditions, i.e. a 24% increase. The efficiency of litter production was estimated as 18.3% in the laboratory and as 16.4% under seminatural conditions. The differences between the two experimental conditions were not significant.