This thesis examines the effect of diet, photoperiod and perceived predation risk on the body composition and energy balance of voles (<i>Clethrionomys </i>sp and <i>Microtus </i>sp).  These rodents are able to regulate their fat mass in response to the changeable environment in which they live by adjusting components of their energy intake and expenditure; thus making them potential candidates for the study of obesity.  Throughout, body composition was determined using both destructive (dissections, Soxhlet) and non-destructive techniques (total body electrical conductivity (TOBEC), morphometrics), whilst energetic variables measured included food intake, assimilation efficiency, resting metabolic rate and daily energy expenditure (DEE). In addition, the usefulness of four non-destructive methods of measuring body composition in voles was assessed and compared to chemical analysis. When fed a diet high in fat (45 %), bank voles (<i>Clethrionomys glareolus)</i> reduced their food intake such that the amount of energy assimilated was equal to that on the control diet and increased fat oxidation to match intake almost immediately.  On a standard diet, these voles gained approximately 10% body mass when switched from short to long photoperiod without corresponding adjustments in energy intake or expenditure, even during the period of most weight gain.  When weasel (<i>Mustela nivalis)</i> faeces was introduced to simulate predation risk, male bank voles reduced the amount of body mass gained in response to photoperiod change, concurrent with a reduction in food intake and an increase in DEE. Both male and female voles exhibited elevated levels of faecal corticosterone when exposed to weasel faeces, indicating increased stress. In a field experiment, male field voles (<i>Microtus agrestis)</i> from sites with low weasel activity were heavier than those from sites with high weasel activity.  There were no differences in body mass between sites with differing vole density.