The stable isotope composition of continuously growing but metabolically inert tissues remains unaltered after they are synthesized, providing insights on seasonal and inter-annual variation in the ecology of individuals. In pinnipeds, isotope analysis of sub-sampled vibrissae can provide a longitudinal record of movement, diet, and even physiological state at the individual level. To reliably apply this approach, however, taxon-specific vibrissa growth needs to be estimated, especially for species with complicated annual life cycles that undergo periods of active foraging interspersed with reproduction and/or molting associated with fasting. Here, we intravenously injected 15 N-enriched glycine to estimate vibrissa growth in 8 free-ranging adult female northern elephant seals during their (shorter) post-breeding and (longer) post-molting foraging trips; all animals were instrumented with satellite tags to track movements and vibrissae were collected when the animals returned to land. We found a significant positive relationship between the maximum δ 15 N values, representing 15 N-glycine injections, and the distance between the origin of the spike and the root of the vibrissa. The δ 15 N spikes that occurred closest to the root were narrower and had lower δ 15 N values than those that occurred closer to the tip of the vibrissa, suggesting differential velocities of 15 N-glycine absorption and vibrissa growth. A derived von Bertalanffy model yielded mean (±SD) growth of 0.015 ± 0.006 d −1 , similar to that previously reported for a single captive northern elephant seal. Vibrissa length is an important consideration for accurately interpreting isotope-based ecological and physiological histories given the non-continuous growth observed in our study.