The endogenous production of carbon monoxide (Vco) in thirty-three diabetic rats induced by alloxan and in fifteen normal rats was measured by serial determinations of the expired carbon monoxide and of the increase of carbon monoxide in the blood during rebreathing in a closed system. Vco (mean ± S.E., µl/250g/hr) in the group of alloxanized rats 24, 48, 72, 96, 120 and over 144 hours after the alloxan injection was 6.2 ± 2.9, 8.4 ± 2.1, 9.2 ± 1.8, 10.7 ± 3.5, 16.4 ± 2.0 and 16.5 ± 2.2, respectively, whereas it was 5.9 ± 0.4 for the control. This indicates that the alloxanized group maintaining a high blood sugar level for over 120 hours has a three-fold higher Vco than the control
