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Exogenous enzymes are proposed as additives in dairy cow diets to increase the digestibility of feed and to improve milk yield. An in vitro experiment was carried out to evaluate the potential of different doses of a new commercial exogenous "-amylase (Ronozyme RumiStar; DSM Nutritional Products, Basel, Switzerland; Novozymes, Bagsvaerd, Denmark) to modulate the organic matter fermentability of corn and barley at two different particle sizes (PS). The two matrices (M) were ground (2 mm sieve) and the resulting grounds separated by size (<750 and >750 μm screen for small and large PS; respectively). Then, samples were incubated for 48h with three enzyme doses (ED; 0, 300 and 1500 kilo novo units/kg feed dry matter for D0, D300 and D1500, respectively) in diluted rumen fluid to measure gas production (GP). Three syringes for each of 12 treatments (2M x 2PS x 3ED) were included in each of two GP runs (replicates). The mixed rumen fluid was from two fistulated dry dairy cows fed with a forage:concentrate (80:20) diet (14.3% CP, 48.5% NDF). One-pool exponential model with Lag was used to describe the in vitro GP kinetics. The GP data (kd: rate of GP, %/h; Vf: final volume, ml; T½: time of half asymptotic, h; and Lag: lag phase, h) were analyzed using a CRD with the MIXED PROC of SAS®. A factorial arrangement was used to study the effects of M (n=2), PS (n=2), ED (n=3) and associated interactions (with run as a random effect). The small particles of corn and barley were fermented faster (p<0.01) than large particles (Table 1). Similarly, the GP dynamics were different between PS, being the Lag and T½ shorter (p<0.01) in small than large PS. However, the Vf was higher (p<0.01) in large than small PS for both matrices. The kd increased when exogenous "-amylase was added (highest values for D1500). However, the increments were related to both M and PS fraction (M x PS interaction, P<0.01). Similar results were obtained for Lag and T½, showing a different dynamic in OM fermen