The gas production technique was developed to determine the fermentation kinetics of organic matter in rumen fluid. However, the gas production technique can be adapted for the determination of protein fermentation characteristics. To do that the buffer must be N-free. All the N coming with the rumen fluid must be incorporated into microbial mass. This can be done by supplying the buffered rumen fluid with an excess of fast fermentable carbohydrates. To prevent a too high input of N from the rumen fluid the rumen fluid can be diluted further compared to the standard 3 times (Cone et al., 1996). This makes N the limiting factor for fermentation and the obtained gas production profiles reflect the availability of N from the feed samples. The aim of the present study was to investigate if the adapted gas production technique is suitable to determine differences in protein availability in rumen fluid. The fermentation characteristics of N of 19 feed samples were determined using the adapted gas production technique. The amount of sample incubated, was that sufficient to provide 15 mg N. The results were compared with data of N degradation obtained with the nylon bag technique.