The link between resting metabolic rate and aging, measured as adult lifespan, was investigated in Drosophila melanogaster by (i) comparing lifespan and metabolic rate of individual flies, (ii) examining the effect of dietary-restriction on the metabolic rate of adult flies, and (iii) comparing the metabolic rate of wild-type and insulin/IGF-1 signalling mutant chico1 flies. The resting oxygen consumption of 65 individually housed and fully fed Drosophila was measured weekly throughout their lifetime. There was no significant difference in the mass-specific rate of oxygen consumption between cohorts that differed in lifespan. Nor was there any statistical correlation between mass-specific oxygen consumption and lifespan of individual Drosophila. The average mass-specific rate of oxygen consumption at 25 degrees C was 3.52+/-0.07 microl O2 mg(-1) h(-1). Variation in mass-specific metabolic rate explained only 4% of variation in individual life span in these flies. Contrary to predictions from the 'rate of living' theory of aging lifetime oxygen consumption was not constant and the lifespan of individual flies accounted for 91% of their lifetime oxygen consumption. An average Drosophila consumes about 3 ml O2 during its adult life. Dietary-restriction had no effect on mass-specific resting metabolic rate both when measured as oxygen consumption by respirometry and when measured as heat production by microcalorimetry. The mass-specific resting heat production of fully fed adult flies at 25 degrees C averaged 17.3+/-0.3 microW mg(-1). Similarly there was no difference in mass-specific metabolic rate of wild-type flies and longliving chico1 insulin/IGF-1 signalling mutant flies, either when measured as oxygen consumption or heat production. Thus, individual variation in lifespan in wild-type flies, and life extension by dietary-restriction and reduced insulin/IGF-1 signalling is not attributable to differences in metabolic rate.