Our objective was to develop a buffer value index that would incorporate alterations in both ruminal fluid pH and buffering capacity as indicators of the influence of dietary buffering and alkalinizing agents on ruminal acid-base status. This index was evaluated using ruminal fluid from four lactating Holstein cows fed either sorghum silage or alfalfa hay in high or low concentrate diets. Ruminal fluid was incubated in vitro for 1, 2, 3, 4, or 5 h with no buffer or with 7.1 g of either NaHCO3, sodium sesquicarbonate, or a multielement buffer added per liter of ruminal fluid. Ruminal fluid pH was lower for diets based on high concentrate or alfalfa; buffering capacity between pH 5 and 7 was greater for high concentrate diets but was not affected by forage type. Ruminal fluid pH was higher for sesquicarbonate than for NaHCO3, the multielement buffer, or the control; however, ruminal fluid H+ concentration was similar between sesquicarbonate and NaHCO3, and both were lower than for the multielement buffer. Hydrogen ion concentration for the multielement buffer was lower than for the control. Buffering capacity was highest for NaHCO3, followed by sesquicarbonate, the multielement buffer, and the control. The buffer value index, which accounted for alterations in both H+ concentration and buffering capacity, was highest for NaCHO3, followed by sesquicarbonate, the multielement buffer, and the control. The poor response to the multielement buffer may be attributable to our relatively short incubation interval (less than 5 h). Dietary buffers increase both ruminal fluid pH and buffering capacity; both of these responses are beneficial.(ABSTRACT TRUNCATED AT 250 WORDS)