Samples of unfractionated forage and isolated NDF from six forages were fermented in vitro, and NDF disappearance and gas and VFA production were measured over time. Rates based on each of these data sets were calculated using a one-pool logistic model. The rates of NDF disappearance and gas and VFA production did not differ within each forage. Gas and VFA production were linearly related to NDF digestion. Gas yield was .35 mL/mg (r2 = .92) of NDF digested for the isolated NDF. The amount of total VFA produced per milligram of NDF digested was more variable than gas (r2 = .72), with a slope of .01 mmol VFA/mg of NDF digested. The relationship between gas and VFA production was linear (mean slope of 1.43 mmol gas/mmol VFA, r2 = .69). The ratios of end products (gas and VFA) to NDF digestion and the ratio of acetate:propionate were variable during the first 8 h of fermentation but changed little after this time. Changes in the acetate: propionate ratio explained 23% of the variation in gas produced per millimole of total VFA detected.