Physiological indicators of muscle catabolism would aid assessment of winter nutritional restriction of ungulates, and urinary 3-methylhistidine has exhibited potential in this regard in several species. We examined the effect of chronic moderate and severe nutritional restriction during winter on urinary 3-methylhistidine:creatinine ratios in seven adult white-tailed deer (Odocoileus virginianus) and the relationship of these ratios to urinary urea nitrogen:creatinine ratios. Mean base line estimates of urinary 3-methylhistidine:creatinine ratio for the control and severely restricted deer (0.043 and 0.086 µmol:mg, respectively) were similar (P = 0.280) and remained unchanged in the control deer throughout the study. In contrast, mean 3-methylhistidine:creatinine ratios increased dramatically as nutritional restriction and cumulative mass loss progressed; the quadratic component of the data for the chronically restricted deer was significant (P < 0.001). Likewise, there was a strong curvilinear relationship (R² = 0.82) between cumulative mass loss (up to 29%) of the pooled deer and urinary 3-methylhistidine:creatinine ratios. Further, urinary urea nitrogen:creatinine ratios were strongly related to 3-methylhistidine:creatinine ratios (r² = 0.89). Our study indicates that further investigation of 3-methylhistidine as an indicator of physical condition and muscle protein breakdown is warranted.