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A method is described for the determination of protein degradation based on measurements of ammonia concentration and gas production (Menke et al. 1979) when a feedingstuff was incubated with rumen fluid in vitro. NH3 liberated during incubation is in part used for microbial protein synthesis. Production of carbon dioxide and methane can be regarded as a measure of energy available for protein synthesis. The ratio, gas production: incorporation of NH3-nitrogen was estimated by addition of starch to the substrate. The response in gas production was linear in the range 0-200 mg starch, when starch was added to 0-200 mg feedingstuff dry matter and 30 ml rumen fluid-medium mixture. Linear regression between NH3-N concentration (y, mg) and gas production (x, ml) yielded an intercept (bo) representing that amount of NH3-N which would be released when no fermentable carbohydrates were available and consequently no bacterial protein synthesis took place. The difference between this intercept bo and NH3-N content in the blank (rumen fluid without substrate added) indicated the amount of NH3 liberated from protein and other N-containing compounds of the feeding-stuff incubated. In vitro-degradable N (IVDN) was calculated as a proportion of total N by the equation: (formula; see text).