Intra-individual variations in carbon (δ13C) and nitrogen (δ15N) isotope measurements of dentine collagen in ungulate teeth can be related to diet and environmental changes at different periods during the life of the animal. A protocol of serial sampling of first, second and third molar roots was applied to modern caribou (17–27 months old) of the Qamanirjuaq herd (Rangifer tarandus groenlandicus), Canada. Based on a previous study, we predicted that M2 would reflect winter, M3 summer and M1 a complete year in terms of the isotopic record. Relatively high δ15N values (ca. 6 to 8‰), previously attributed to winter stress, were found in different molars of different specimens, reflecting a period of growth between April 1966 to April 1967. Previous results on other teeth from the same population confirmed that a high δ15Ncoll value signal corresponded to the winter of 1966/67. This temporary increase in δ15N value was probably linked to diet and/or environmental change. Collagen from M1 reflects the first winter whereas M2 and M3 reflect the second winter of life of young caribou. A longer time record including summer is represented by the bone collagen of the mandible. Results obtained on molar roots and mandible bone of the modern caribou of Banks Island herd (Rangifer tarandus pearyi) confirmed this seasonal record. Such collagen isotopic analysis on M1, M2 and M3 roots and jawbone can be applied to reindeer found in archaeological sites. Mandibles retaining deciduous premolars are preferable to avoid the possible loss of the winter tooth signal observed in animals older than 2 years. Copyright © 2010 John Wiley & Sons, Ltd.