In a cross-over study with adult cats the effect on nitrogen (N) excretion of a diet supplemented with fructooligosaccharides (FOS) was compared with a diet supplemented with an equal amount of fructose and glucose. FOS raised N excretion with faeces by 26% (P < 0.05) and non-significantly lowered N excretion with urine by 5%. Thus, there was a shift in N excretion from urine to faeces. The amount of faecal dry matter was significantly higher (by 23.3%) when FOS was consumed.