The aerobic capacity model proposes that endothermy is a by-product of selection favouring high maximal metabolic rates (MMR) and its mechanistic coupling with basal metabolic rate (BMR). Attempts to validate this model in birds are equivocal and restricted to phenotypic correlations (r(P)), thus failing to distinguish among- and within-individual correlations (r(ind) and r(e)). We examined 300 paired measurements of BMR and MMR from 60 house sparrows before and after two levels of experimental manipulation - testosterone implants and immune challenge. Overall, repeatability was significant in both BMR (R=0.25 +/- 0.06) and MMR (R=0.52 +/- 0.06). Only the testosterone treatment altered the rP between BMR and MMR, which resulted from contrasting effects on r(ind) and r(e). While r(ind) was high and significant (0.62 +/- 0.22) in sham-implanted birds, re was negative and marginally non-significant (-0.15 +/- 0.09) in testosterone-treated birds. Thus, the expected mechanistic link between BMR and MMR was apparent, but only in birds with low testosterone levels.