Two pairs of adult castrated male sheep were given intraruminally 2 g/kg body weight per day of sodium n-butyrate rapidly 10 s at noon or slowly over 20 to 24 h from noon in four trials in which they received only mixed minerals and water ad libitum. Their ruminal papillae were biopsied just before each administration and 24 h after the final administration. Mitotic indices of the epithelial cells were observed. Mitotic indices were lower than .48% before butyrate administration. In sheep rapidly administered butyrate, mitotic indices increased to 1.29%, 2.33%, 2.65%, and 1.84% in 1 day after the first administration in four trials and tended to decline on later days. In sheep slowly administered butyrate, mitotic indices stayed lower than .50% and did not show significant fluctuations. Our data support our hypothesis that an increase in the rate of intraruminal production of volatile fatty acids promotes proliferation of epithelial cells of the organ.