The effect of starch fermentation in the rumen on the kinetics of roughage digestion, was studied using 12 sheep fed three roughages, viz. lucerne hay, maize cob leaves and wheat straw. The amount of starch infused per day was increased from o to 600 g/d in steps of 20 g/d over 30 days. The amount of starch infused was delivered at two rates; either as the daily amount infused over 12 h, or over 24 h. The well known negative effect of starch fermentation on roughage intake and digestion was observed when the lucerne and maize cob leaf diets were fed, but not when the wheat straw diet was used. The rate at which starch was infused affected the intake of maize cob leaves in a variable manner. The slow infusion rate led to a very small negative effect, whereas the fast rate of starch infusion resulted in a large negative effect. Regardless of diet, the negative effects of starch on intake could not be ascribed to reduced rumen fill, nor to a reduced concentration of rumen ammonia. Furthermore, pH of the rumen contents was not lowered. Rate of passage of non-fermentable OM was decreased by starch infusion on one diet (maize cob leaves) only, even though the mean retention time (MRT) of water was significantly influenced by starch infusion on all the diets. Although starch fermentation negatively affected many aspects of roughage digestion, the paramount factor appears to be a reduced rate of roughage digestion.