The luminal metabolism of urea was studied using double-labelled urea ([15N2]urea) which was placed in the lumen of the colon through a colostomy. The recovery of label was measured as [15N2]urea or [14N,15N]urea in urine and as 15N in stool. Five patients with a loop colostomy allowed a comparison of the right functioning colon with the left defunctioned colon in the same individual. Five subjects with a left end-colostomy enabled a comparison of the right with the left functioning colon. A significantly greater proportion of labelled urea was recovered as [15N2]urea in the urine when the dose was placed in the left defunctioned colon (29%) compared with either the left or the right functioning colon (9 and 4%, respectively). This is interpreted as being a result of a decrease in the bacterial activity and concomitant urea hydrolysis in the defunctioned colon. On average more than half of the label was retained in the body, regardless of whether the urea was placed in the funtioning or the defunctioned colon, on the left or on the right. The data confirm that the colon is permeable to the intact urea molecule. Intraluminal urea is readily hydrolysed in the functioning colon. A large proportion of the nitrogen released by urea hydrolysis may be retained within the metabolic nitrogen pool of the host. There are significant differences in the handling of urea nitrogen in the defunctioned colon relative to the functioning colon.