We examined whether or not changes occur in the quantity of carbon monoxide (CO) production as a consequence of the intake of ordinary meals. The study was performed by following diurnal changes in the concentration of CO in the breath over approximately 12 hr after each meal on 17 healthy subjects. CO is a product of the reaction of the enzyme heme oxygenase-1 (HO-1) induced through oxidative stress. The average value for diurnal variations in the concentration of CO in the breath was 1.9 ± 0.5 (0.8 to 3.3) ppm, and the maximum range of variation in individual cases came within the narrow range of 0.4 to 1.3 ppm. There were no significant differences in average values before and after meals. The value for the total quantity of CO in the breath 12 hr sought on the basis of the area under the diurnal variation curve was 22.8 ppm (0.39 mmol). This figure was very close to the heme breakdown quantity of 0.35 mmol (theoretical value, corresponding to the CO production quantity) every 12 hr.