IT has been shown that in 4 out of 5 test subjects carboxytolbutamide [1-butyl-3-p-carboxyphenylsulphonylurea] was excreted in the urine by a first-order process following the oral or intravenous administration of tolbutamide [1-butyl-3-p-tolylsulphonylurea]1; furthermore, that the rate constant found for this excretion was the rate constant for the in vivo oxidation of tolbutamide to carboxytolbutamide. An average of about 92 per cent of 465 mgm. oral doses of tolbutamide was recovered in the urine as carboxytolbutamide in 36 hr. from the four subjects that showed first-order excretion of the latter material.