- We investigated the urinary cortisol and urea nitrogen responses of mule deer (Odocoileus hemionus) to winter population densities. We collected urine samples from 14 hand-reared mule deer fawns and 14 hand-reared mule deer yearlings placed in 2 pastures (66 and 169 ha) stocked with wild deer at high (133 deer / km2) and low (44 deer / km2) densities, respectively. Levels of urinary cortisol: creatinine ratios increased exponentially as winter progressed and with pasture density and were correlated with wild fawn survival in the pastures. Animals larger at the start of winter had lower cortisol: creatinine ratios during the winter. Weather conditions and date of birth for fawns, and snow depths for yearlings, exerted minor effects. Urea nitrogen: creatinine ratios declined as winter progressed and increased with cortisol levels. Rate of mass loss increased over time in fawns, was constant in yearlings, and was weakly correated to cortisol: creatinine ratios. Urine cortisol, assumed to reflect energy deficit, allows researchers to distinguish high levels of urea nitrogen caused by the availability of crude protein from those caused by muscle catabolism. By reflecting both environmental and animal condition, urine cortisol provides a tool for assessing population condition and ecological density.