Energetic requirements during reproduction are important determinants of the onset of reproduction and of breeding strategy (e.g., breeding post-partum) and therefore affect female reproductive output in seasonally varying environments. To balance the energetic needs of breeding with energy availability, females must optimize energy allocation between their own energy use and energy allocated to their litter. Here, we studied energetic costs and potential energetic trade-offs of reproduction in female bank voles (Clethrionomys glareolus). We measured energy consumption, i.e., metabolic rates as determined from carbon dioxide production of females either with their pups (breeding unit) to find the total energy expenditure of breeding or alone (maternal energy use), to find potential trade-offs between energy allotted to offspring and to maintenance metabolism. We found that energy used by breeding units remained on the non-pregnant level during pregnancy and increased during lactation but did not increase further during post-partum pregnancy. Maternal energy use remained unchanged through the breeding cycle. Carbon dioxide produced per gram of tissue decreased throughout the reproductive cycle for both breeding units and females alone, suggesting that energy use efficiency improved with advancing pregnancy and lactation. Our study supports the idea that in small mammals reproduction does not increase female energetic costs beyond the costs involved in increasing body mass. We found further that offspring body mass did not differ between lactating non-pregnant and lactating pregnant females and that there were no trade-offs between female energy use and offspring body mass. We conclude that energy allocation by breeding iteroparous females allows for an optimal breeding strategy, i.e., fertilization of new ova post-partum.