The present study was carried out to investigate the effect of ruminal infusion of acetic acid on rumination behaviour in sheep fed on the hay diets with varied crude protein (CP) content. Italian ryegrass, timothy and lucerne hays were used and their CP content was 7.3,10.4 and 14.9, respectively. Acetic acid with buffer, total amount of 1,000 ml, was infused at a rate of 62 mmol per hour (300 kcal/day) into the rumen of wethers fed 2.0% of dry matter (DM) of body weight per day. The obtained results were as follows: 1. the digestibility of organic matter and nitrogen-free extractives (NFE) tended to decrease with the infusion of acetic acid regardless of the hay diets-CP content consumed, but not significantly (P>0.05); 2. time spent eating was significantly (P<0.05) prolonged with acid infusion in sheep fed on timothy or lucerne hays, and consequently, rate of eating was also significantly (P<0.05) slow, although there was no significant (P>0.05) effect of acid infusion in sheep fed on Italian ryegrass hay; 3. daily time spent ruminating and daily number of boli in sheep fed on timothy or lucerne hay also tended to increase (P<0.05) after infusion of acid, though there was no changes in sheep fed Italian ryegrass hay with ruminal infusion of acetic acid; 4. cyclic rate was significantly (P<0.05) shorter in sheep fed lucerne hay with acid infusion than in control, whereas it was almost the same in sheep fed on Italian ryegrass or timothy hay with or without acid infusion; 5. the rumination index was also lengthened (P<0.05) after acid infusion in sheep fed on timothy or lucerne hay, though in sheep fed on Italian ryegrass hay with acid infusion had no effect on rumination index. From these results, it can be concluded that the effect of ruminal infusion of acetic acid on fibre digestibility of hay diet could have a much more strong influence on the control to microbial activity than on comminuting the feed particles through remastication during the ruminating period.