The capacity of ruminal bacteria to regulate H+-ATPase synthesis in response to reduced pH was investigated to explain acid tolerance. The activity of H+-ATPase in Streptococcus bovis, an acid-tolerant bacterium, was 2.2-fold higher at pH 4.5 than at pH 5.5. The increase in the amount of H+-ATPase protein was similar, suggesting that the increase in H+-ATPase activity is owing to the increase in H+-ATPase synthesis. The level of atp-mRNA at pH 4.5 was 2.5-fold higher than at pH 5.5, indicating that H+-ATPase synthesis is regulated at the transcriptional level, responding to low pH. In Ruminococcus albus, an acid-sensitive bacterium, H+-ATPase activity, the amount of H+-ATPase protein, and the level of atp-mRNA at pH 7.0 were similar to the values at pH 6.0, the lowest pH permitting growth. This result suggests that R. albus is incapable of enhancing H+-ATPase synthesis at low pH. Thus, acid tolerance appeared to be related to the capacity to augment the synthesis of H+-ATPase responding to low pH.