An experiment, to investigate the effect of changes in ruminal levels of fermentation products, on eating and rumination behaviour related to feed utilization, acetic and propionic acids, which, provided 300 kcal/d/head, is reported. Two sheep fed a hay diet alone were continuously infused, VFAs into the rumen in a crossover design over 12 days period. The average daily eating time and, rumination appearance (lag time after eating) tended to be prolonged with infusions of both acids, but not significantly. Daily time spent ruminating was almost similar in VFAs infused and control, groups. Apparent digestibility of organic matter, crude protein and crude fibre were fairly reduced, after infusion of acetic acid, although there are no changes in nutrient digestibility after propionic, acid infusion. Urinary nitrogen excretion was markedly reduced, and consequently, the nitrogen, retention was clearly increased after VFAs infusion, and then in particular, the retained nitrogen, was significantly more after acetic acid infusion than that in control. The blood glucose level was, markedly increased after propionic acid infusion, though there was no change in blood glucose level, in acetic acid infusion, From these results, it might be suggested that acetic acid or propionic acid infused at physiological, level into the rumen of sheep fed on hay diet alone clearly improve the utilization of dietary nitrogen, whereas there are no clear effect on the eating and/or rumination behaviour in sheep.