The study investigated in vitro simulated rumen metabolism of freshly cut and wilted cocksfoot [Dactylis glomerata; high polyphenol oxidase (PPO)] and tall fescue (Festuca arundinacea; low PPO). A 16-vessel RUSITEC was used with the four treatment combinations: cocksfoot wilted (C(w)); cocksfoot fresh (C(f)); tall fescue wilted (TF(w)) and tall fescue fresh (TF(f)). Rumen liquor was collected from four fistulated dairy cows maintained on permanent pasture. The experiment ran for 12 d with sampling of effluent for rumen parameters [volatile fatty acids (VFA) and NH(3)-N] on days 10 and 11 at 0, 1, 2, 3, 4, 6, 8 and 24 h intervals. On days 9-12, gas production and composition were measured using a plastic gas collection bag attached to the effluent bottle. At the end of day 12, the grass residue and effluent from the vessels were collected and analysed for digestibility and lipid fractionation. Lipolysis was calculated as the proportional loss of glycerol-based membrane lipid between the forage and residue. Polyphenol oxidase was higher (P < 0 center dot 001) in C(f) than the other treatments; C(w) was higher than both TF treatments, with no difference between TF(w) and TF(f). The level of protein-bound phenol (product of oxidation reaction) tended to be higher for C(w) and TF(w) than C(f) and TF(f), (P < 0 center dot 1) and was higher for cocksfoot than tall fescue (P < 0 center dot 001). As an average across the day, NH(3)-N was lower (P < 0 center dot 001) in cocksfoot than tall fescue, despite the lower nitrogen concentration of tall fescue, and in fresh as opposed to wilted grass (P < 0 center dot 001). There was a trend (P < 0 center dot 1) for lipolysis to be lower in cocksfoot than tall fescue, and for both wilted treatments to be lower than the fresh (P < 0 center dot 05). Total VFA concentration was not different across treatments, although there were differences in molar proportions of individual VFA. Cocksfoot as opposed to tall fescue showed a small effect in lowering lipolytic and proteolytic activity (release of NH(3)-N), although other differences between the grasses other than just PPO activity such as lipase activity, diphenol substrate content and digestibility may have greater effects between the grasses confounding the effect of PPO.