Rationale Nitrogen stable isotope ratio (δ 15 N) processes are not well described in reptiles, which limits reliable inference of trophic and nutrient dynamics. In this study we detailed δ 15 N turnover and discrimination (Δ 15 N) in diverse tissues of two lizard species, and compared these results with previously published carbon data (δ 13 C) to inform estimates of reptilian foraging ecology and nutrient physiology. Methods We quantified 15 N incorporation and discrimination dynamics over 360 days in blood fractions, skin, muscle, and liver of Sceloporus undulatus and Crotaphytus collaris that differed in body mass. Tissue samples were analyzed on a continuous flow isotope ratio mass spectrometer. Results Δ 15 N for plasma and red blood cells (RBCs) ranged between +2.7 and +3.5‰; however, skin, muscle, and liver did not equilibrate, hindering estimates for these somatic tissues. 15 N turnover in plasma and RBCs ranged from 20.7 ± 4 to 303 ± 166 days among both species. Comparison with previously published δ 13 C results for these same samples showed that 15 N and 13 C incorporation patterns were uncoupled, especially during winter when hibernation physiology could have played a role. Conclusions Our results provide estimates of 15 N turnover rates and discrimination values that are essential to using and interpreting isotopes in studies of diet reconstruction, nutrient allocation, and trophic characterization in reptiles. These results also suggest that somatic tissues can be unreliable, while life history shifts in nutrient routing and metabolism potentially cause 15 N and 13 C dynamics to be decoupled.