Questions about the adaptive importance of metabolic rate can be approached only when measurements of differences between individuals are repeatable. We made daily measurements of CO(2) production, body mass, and food uptake over the adult life span of unmated Vanessa cardui kept under constant environmental conditions in both fed and unfed treatments. Mass and CO(2) production generally declined with age in both treatments, though with much day-to-day variability in the fed treatment. For the full samples, metabolic rate was repeatable for the unfed treatment (repeatability r = 0.60) but not for the fed treatment (r = 0.03). Differences between fed and unfed individuals of the same age range were repeatable for the unfed treatment (r = 0.39) but not for the fed treatment (r = -0.20). Removing age effects on CO(2) production yielded still higher repeatability in the unfed treatment (r = 0.83), though not in fed butterflies of the same age range (r = -0.02). However, repeatability of CO(2) production of fed butterflies increased sharply with age, rising to 0.82 for butterflies age 8-10 d. Although food uptake mass was repeatable (r = 0.52), feeding history explained little variation in CO(2) production. We conclude that for V. cardui and possibly for other insects of similar feeding habit, variation in metabolic rate between individuals is best represented by measurements of unfed individuals of the same age or of older fed individuals.