Mature wether sheep were continuously given a concentrate-type diet and intraruminally infused with treatments of sodium bicarbonate (NaHCO3) at 0, 20 or 40 g/l per day according to a 3 x 3 Latin-square design. No effects of NaHCO3 on digestive parameters were statistically significant (P greater than 0.1). However, NaHCO3 additions decreased dietary intake and at 20 g/day supplementation, apparent digestibilities of dry matter, organic matter and nitrogen were increased, but 40 g/day bicarbonate reduced these values. Additions of NaHCO3 tended to increase rumen pH values, buffering capacities, concentrations of ammonia and acetate: propionate ratios. Water intakes, rumen volumes and population densities of rumen bacteria and protozoa were also higher with NaHCO3 supplementation, but these were not reflected by consistent changes in either rumen dilution rates or rumen microbial nitrogen concentrations. Interrelationships of digestive parameters that might be produced by the neutralising, buffering and osmotic pressure effects of sodium bicarbonate are discussed.