A stepwise adaptation experiment was performed on 5 adult sheep fed ad libitum on successive diets containing 10; 26,5; 43,5; 54 to a final diet containing 65% maize meal and molasses for ll9 days. All diets contained a mixed buffer and l5% CP. Samples of ruminal ingesta were taken at short intervals during the first l4 h of the feeding cycle until Day 7 on the final diet when it was continued over the entire 24 h. Concentrations of ruminal NH3-N were mostly higher than 6 mM and therefore above the limiting concentrations (3,4 – 5 mM) for growth of rumen bacteria. Ruminal lactic acid accumulated transiently about ½ – 2 h after each of the 2 daily feeds. The peak concentration (l – 10 mM) showed a general tendency to decrease as the adaptation progressed. Higher than usual peak values occurred between 22 – 71 days on the final diet indicating a tendency towards unstable conditions in the rumen. This was counteracted by a voluntary reduction in food intake by the sheep which allowed stable conditions to return as indicated by a decrease in the peak lactic acid values. The mixed dietary buffer was most effective since, despite high food intakes of up to 2,65 kg/d, the ruminal pH rarely fell below 5,5. and this was mostly for short periods of time (av. 2,l h, range 1 – 6 h) unlikely to upset the balance of the ruminal flora. The incidence of urinary calculi could be attributed to a high intake of P supplied by the fishmeal supplement and dietary buffer.