Sucrose and lactose are used as energy supplements in ruminant diets. In our previous study (Hussain and Miller, 1998) lactose maintained a higher rumen pH, increased organic matter and neutral detergent fibre digestion in the rumen, reduced the number of rumen protozoa and increased microbial (bacterial) protein flow to the duodenum compared with sucrose. However, it was not clear whether the effect of lactose on pH or on reduction of protozoa was the main factor increasing rumen fermentation and microbial protein flow. The objective of the present study was to examine the effect of isoenergetic amounts of sucrose and lactose as supplements (equivalent to 50 g glucose) on rumen fermentation and metabolism in the presence of sodium bicarbonate to maintain rumen pH.