We evaluated the relationship between organ mass and the limits to aerobic metabolism in house sparrows Passer domesticus. The results were used to test three models of performance limitation (the central limitation, peripheral limitation and symmorphosis concepts). Basal metabolic rate (BMR) was determined during the rest phase. The maximum rate of oxygen consumption during exercise (V��(O2max)) was measured in an enclosed wheel that allowed limited hovering flight. Neither BMR nor V��(O2max) was affected by gender, but adults had significantly higher V��(O2max) and lower BMR than juveniles. The masses of most central organs (gut, gizzard, liver, heart, kidney and reproductive organs) differed significantly between ages. There were no gender differences in organ mass among juveniles, but liver mass differed between male and female adults. In the pooled data, BMR was positively correlated with the mass of three central organs (gut, liver and kidney) and with one peripheral effector (breast muscle); together, these explained more than half the variance in BMR (r2=0.57). In adults, BMR was positively correlated with the mass of reproductive tissue. The masses of one peripheral effector (breast muscle) and one central organ (the heart) were positively correlated with V��(O2max) (r2=0.17 for the pooled data set). These results are consistent with a symmorphosis model of aerobic capacity. We found a significant positive relationship between BMR and V��(O2max) in juveniles, but not in adults. Taken together, our data indicate that house sparrows can achieve elevated V��(O2max) without paying a 'penalty' (fitness trade-off) in the form of an increased BMR.