Previous studies suggest that a nutritional/energetic state may be determined from the δ15N-values of an animal's tissues. It has been hypothesized that if an animal is in a negative nutritional/energetic balance, 15N will be retained by the tissues preferentially and 14N preferentially excreted. Over time, this should cause enrichment of the δ15N-values of the tissues. Tests of this hypothesis have met with mixed results. To date, these studies have focused on endotherms. Few studies have considered the effects of fasting on the 15N/14N ratios of substrates from ectotherms. We hypothesized that the δ15N-values of tail tissue and uric acid of fasted lizards would increase over time, while that of the fed lizards would remain the same. We fasted five side-blotched lizards (Uta stansburiana) and five green anoles (Anolis carolinensis) for 14 days. At the same time we fed six side-blotched and four green anoles ad libitum. All lizards were given water ad libitum. During this period, we collected uric acid from each lizard daily and tail tissue on the first day of the study and on the last day of the study. We found the the δ15N-values of uric acid of fasted lizards at the end of the study were 15N-enriched compared with that of the control lizards in both species, probably due to a shift in amino acid source pools during fasting. However, we found no difference between treatment and controls in the δ15N-values of the tails. Copyright © 2007 John Wiley & Sons, Ltd.