Rates of turnover of oxygen, water and sodium were measured in free-ranging and captive Lacerta viridis in western France during their period of reproduction in May, and again in late autumn prior to their entering winter hibernation. Rates of C02 production and turnover of water and sodium in captive lizards showed little significant variation and these animals appeared to be buffered to a large extent from environmental changes, even though they were exposed to external climatic influences. Overall means for rates of CO2 production in free-ranging individuals did not vary statistically between seasons but considerable individual variation was observed during the breeding season in May. The animals could be divided into 'active' and 'inactive' groups on the basis of their rate of CO2 production and other significant differences were apparent between these two groups: 'active' individuals had increased rates of sodium turnover, lower body water contents and lost body mass during the period of observation whereas the 'inactive' individuals gained mass over the same period. These data suggest that not all individuals necessarily participate in the activity associated with breeding in spring in this species and further behavioural studies would seem appropriate in the light of these physiological data. Calculation of the 'Water Effectiveness Index' for this species from the ratio of water to energy turnover suggests that Lacerta viridis is a profligate utiliser of available resources when compared with similar-sized species of lizards inhabiting desert environments.