Four sheep each fitted with a rumen and duodenal cannula, were given a diet of lucerne silage and molasses (870 and 150 g dry matter (DM) day−1, respectively). Four experimental treatments consisting of a control (C), protein feed (P) (88 g DM day−1), sodium bicarbonate (B) (50 g DM day−1), or a combination of sodium bicarbonate and protein (PB) were added to the diet in a 4 × 4 Latin square. Sodium bicarbonate increased rumen pH and decreased rumen ammonia levels: pH values were 6.63, 6.54, 6.83 and 6.91 (SE = 0.09), and NH3N levels were 140, 174, 109 and 139 mg l−1 (SE = 5.9) for treatments C, P, B and PB, respectively. The fractional turnover rate of rumen liquid was increased by sodium bicarbonate alone or in combination with protein; turnover rate was 0.088 h−1, 0.094 h−1, 0.118 h−1 and 0.131 h−1 (SE = 0.006) for C, P, B and PB, respectively. Duodenal passage of microbial nitrogen was increased by both protein and bicarbonate separately and, to a greater extent, by the combined treatment; values were 9.2 (C), 11.0 (P), 15.7 (B) and 20.4 (PB) g day−1 (SE = 3.3). The apparent efficiency of microbial protein synthesis was significantly (P <0.05) increased by bicarbonate but not by protein; values expressed in grams microbial nitrogen per kilogram organic matter apparently digested in the rumen, for C, P, B and BP respectively, were 16.0, 16.6, 26.2 and 29.7 (SE = 4.0).