Rumen fermentation was studied during three years in lactating Dorper and Merino ewes, supplemented daily with crude protein and/or energy via rumen fistulae on veld. Herbage, collected with oesophageally fistulated wethers, and maize meal were incubated in sacco in the rumen of the ewes. Four distinct observations were made in terms of in sacco dry matter (DM) disappearance of herbage, namely: (i) a significant (P ≤ 0. 05) difference between breeds in winter; (ii) an increase of 100% from winter to summer; (iii) a significant (P ≤ 0.05) increase due to protein supplementation in winter; and (iv) a non-significant (P ≤ 0.05) tendency to decrease in summer with increasing levels of energy supplementation. None of these differences or tendencies were evident in the in sacco DM disappearance of maize meal during any of the trial periods. Results are discussed in relation to changes in rumen pH and rumen ammonia concentration, as well as feed intake by the ewes, in the same trials.