Two trials were conducted to investigate various aspects of the production and metabolism of acetate, propionate, and butyrate. Two Holstein steers, each serving as its own control, were used in each trial. The four steers weighed between 223 and 307 kg. Each steer was fed 4.6 kg/day of 70% chopped alfalfa/30% cracked corn, with or without 150 mg monensin, at 2-hour intervals. In trial 1, U-('14)C acetate, U-('14)C butyrate, and 1-('14)C propionate were used to measure plasma acetate irreversible loss (IL), rumen propionate IL, and the production and interconversion rates of rumen acetate and butyrate. In trial 2, uniformly ('14)C labeled acetate, propionate, butyrate, glucose, and bicarbonate were used to solve a five compartment model. This model included rumen acetate, propionate, and butyrate; and plasma acetate and blood bicarbonate;The five compartment model from trial 2 showed no monensin effects and was therefore analyzed without regard to treatment. Flow to compartment x from compartment y is designated F(,xy). The five compartments are designated as follows: a = acetate, p = propionate, b = butyrate, g = glucose, c = bicarbonate, and o = outside of the model. The transfer rates were (mol C/day): F(,og) = 2.5, F(,go) = 7.0, F(,gp) = 2.8, F(,ga) = .7, F(,gb) = .4, and F(,gc) = 2.6; F(,op) = 1.5, F(,po) = 7.2, F(,pg) = 0, F(,pa) = .3, F(,pb) = o, and F(,pc) = 1.1; F(,oa) = 5.2, F(,ao) = 18.5, F(,ag) = 0, F(,ap) = .3, F(,ab) = 2.2, and F(,ac) = .2; F(,ob) = 1.0, F(,bo) = 7.2, F(,bg) = 0, F(,bp) = 0, F(,ba) = 2.0, and F(,bc) = 0; F(,oc) = 92.5, F(,co) = 62.8, F(,cg) = 10.9, F(,cp) = 4.1, F(,ca) = 13.0, and F(,cb) = 5.6. The results of both trials were combined to measure the effect of monensin on production of acetate, propionate, and butyrate. Three of the four steers responded by increasing propionate production at the expense of acetate production. For these three steers, propionate production increased from 7.5 to 10.8 mol C/day and acetate production decreased from 19.2 to 15 mol C/day (p < .05). Monensin had no significant effects on butyrate production, interconversion of acetate and butyrate, or the total hexose fermented;Using data from all four steers, the following regression was calculated: Propionate production = 26.3-.98* Acetate production (r = -.935, p < .001). Total VFA production was equivalent to the fermentation of 7.5 moles of hexose per day.