Grass samples were harvested during the 1993 growing season after a precut on April 27, 1993 and were stored frozen or left to ensile in 30-L buckets. Effects on chemical composition and fermentation kinetics of the maturation of the grass and of ensiling were investigated. Chemical composition and fermentation kinetics were determined using the gas production technique, in vitro techniques, and the nylon bag technique. Two silage samples were also investigated in vivo. Maturation caused a decrease in crude protein content and organic matter degradability and an increase in neutral detergent fiber, acid detergent fiber, and lignin contents. Degradation rates were highest for the youngest samples and decreased as grass and silage matured. This result could be observed from the first derivative of the cumulative gas production curves (i.e., the rate of gas production) and from the gas production parameters. The rate of degradation of the nonsoluble fraction of the young samples, determined using the gas production technique, was relatively higher in rumen fluid from a cow fed silage from grass cut at a young stage. More mature samples were degraded relatively faster in rumen fluid from a cow fed mature grass silage, suggesting a specific adaptation of the rumen microorganisms to the grass properties. There was a good relationship among the second phase of gas production (i.e., fermentation of the nonsoluble fraction), maturity of the grass and grass silage samples, degradability determined with the Tilley and Terry technique, and degradability determined after 46 h of incubation in rumen fluid. Results obtained with both of the different in vitro techniques and the nylon bag technique were confirmed by the in vivo experiments involving the two silage samples.