High metabolic rate may provide fitness benefits for individuals. But high metabolic rates incur energetic costs and the need to ingest more food, increasing the risks of ingesting harmful substances from the environment. How organisms respond to elevated ionizing radiation is an important question in the light of pollution from nuclear accidents and waste, and reliance on radiation for medical treatments. Within and around the Chornobyl exclusion zone, we investigated how the bioenergetics of wild rodents inhabiting a gradient of radioactive contamination from ‘clean’ (<15.4 μGy day⁻¹) to contaminated (50–2400 μGy day⁻¹) affects their biological burden of radionuclides. We found that the biological radiation dose negatively correlates with aerobic metabolic scope (high self-maintenance and low aerobic capacity) in adults but positively correlates with metabolic scope (high aerobic capacity) in subadults. These findings suggest that metabolic downregulation may contribute to protection against radionuclide exposure, but that it is constrained by developmental obligations. The results also suggest detrimental effects of radiation on animal physiology. Understanding the physiological mechanisms underlying these relationships will be key for risk assessment of environmental contamination, radiotherapies and space exploration, and may help to rectify discordant opinions concerning the effects of radiation on organismal ecology.