Two experiments were conducted to investigate the effect of ruminal infusion of volatile fatty acids (VFAs) on eating and rumination behaviour and feed utilization in sheep fed on hay. The amounts infused were 300,300 and 150 kcal per day per animal of acetic, propionic and butyric acids, respectively. Each acid solution was continuously infused into the rumen of sheep fed on timothy hay alone (Experiment I). In Experiment II, sheep fed on roughage with varied crude protein (CP) levels (6.0, 7.3, 9.1 and 15.5% DM). Four sheep, two in each, were used in repeated crossover designed experiments. Daily eating time was lengthened by infusion of acetic and propionic acids as compared to control. There was no change in eating time after infusion of butyric acid (Experiment I). Daily eating time increased after propionic acid infusion and change in CP contents of the basal diet compared to control sheep. There was no change in eating time after infusion of acetic acid (Experiment II). Rumination appearance tended to shorten after infusion of acetic and propionic acids as compared to that of control. Butyric acid infusion gave opposite (Experiment I). Rumination appearance was long with an increase in dietary CP, but was not changed by ruminal infusion of volatile fatty acids (Experiment II). Daily rumination time tended to be long after infusion of butyric acid and short after infusion of acetic acid, and did not change after infusion of propionic acid (Experiment I). Cyclic rate tended to lengthen after infusion of acetic acid as compared to that of control, but there was no change after infusion of propionic and butyric acids (Experiment I). Cyclic rate was shortened after infusion of acetic acid regardless of the CP content in the diet compared to that of control. Ruminal infusion of propionic acid had no clear effect on cyclic rate in sheep fed on diets with varied CP contents (Experiment II). Rumination appearance was fairly changed with varying levels dietary CP content regardless ruminal VFAs infusion. It was therefore, concluded that ruminal infusion of VFAs had little effect on eating and rumination behaviour and feed utilization. It was further concluded that chemical stimulus rather than physical stimuli, such as the increment of ruminal nitrogen could influence the chemoreceptor on the rumen wall.