The effect on urea metabolism by the supplementation of oligofructose to a reduced protein diet was evaluated in cats by the use of labelled urea. The effect on faecal odour was also evaluated. Four cats were tested in a crossover study with two treatments: control and fructo-oligosaccharide (FOS). The FOS was supplemented at 3.11% on dry matter (DM) basis to a reduced protein diet (28.9% DM). After an adaptation period of 3 weeks, faeces and urine were collected during a 5-day collection period. Fresh faecal samples were collected for determination of odour components. On the first day of the collection period, labelled urea was injected subcutaneously. Urine production was estimated by potassium excretion. The fresh faecal samples were incubated in an air-closed recipient with two solid-phase micro extraction (SPME) fibres to bind the produced sulphur (S)-containing components. The reduced protein diet decreased plasma urea concentration but FOS supplementation had no effect. The tendency for a higher faecal output by FOS supplementation was the consequence of both an increased moisture content and faecal DM production. Supplementation of FOS showed tendencies to increase total faecal nitrogen (N) excretion and faecal (15)N excretion and tended to decrease urinary (15)N excretion. Twenty-seven different odour components were detected but were not affected by FOS supplementation.