To study the effect of lactulose on the route of nitrogen excretion, we fed six healthy, adult dogs on diets containing either 0, 1 or 3 g lactulose/MJ metabolizable energy according to a 3 x 3 Latin square design. The results were analysed to identify statistically significant linear trend effects of lactulose. Faecal pH was significantly lowered by lactulose. Faecal ammonium and nitrogen excretion tended to be raised by lactulose feeding whereas urinary urea excretion was significantly reduced. Lactulose feeding significantly lowered apparent nitrogen digestibility. It is concluded that lactulose feeding shifts nitrogen excretion from urine to faeces in dogs which may be beneficial for liver patients. The data are in line with the concept that lactulose stimulates bacterial growth in the colon which in turn enhances faecal nitrogen excretion and lowers the entry of colonic ammonia into the bloodstream, leading to a lesser workload for the liver and less urinary nitrogen excretion. Lactulose consumption was also found to produce a dose-dependent increase in the apparent absorption of calcium and magnesium, but not phosphorus.