The aim of this experiment was to distinguish between the effects of dietary roughage: concentrate ratio and rumen ammonia concentration on rumen in situ degradation in sheep. Three diets with roughage: concentrate ratios of (Eragrostiscurvula hay:maize) 75:25, 50:50 or 25:75 were used as basal diets, and three rumen ammonia concentrations (6, 12 or 15 mM) were imposed on each of these. The amounts of supplemental urea required to achieve these rumen ammonia concentrations were determined from regression equations relating urea supplementation level to rumen ammonia concentration for each diet. Rumen ammonia concentration had no effect on nitrogen (N) or dry matter (DM) degradation rates of any of the feedstuffs studied ( P >0.05). E. curvula hay was depressed by 48% (P < 0.05) by increasing dietary maize content, but the N degradation rates of lucerne hay, maize, sunflower oilcake and cottonseed oilcake were not affected (P > 0.05). DM degradation rate of E. curvula hay was decreased by 58% by increasing dietary maize content, and that of lucerne hay by 44% (P < 0.05). DM degradation rates of maize, sunflower oilcake and cottonseed oilcake were not affected (P > 0.05). It was concluded that, although the potential degradative activity of rumen contents is influenced by the ratio of roughage in the diet, the effect of these changes differs between feedstuffs. Expression of basal diet effects appears to be a result of several factors which include the potential rate and extent of degradation of the test feedstuff and the prevailing rate of flow of particulate matter from the rumen.