1. The objective of the present experiment was to study the effects of oak (Quercus incana) leaves rich in tannins on various enzyme activities of the bovine rumen. 2. The procedure employed was incubation of tannin-rich, very-low-tannin or virtually tannin-free leaves in nylon-gauze bags in the rumen, and determination of enzyme activities in microbes tightly bound to the solid matrix and in microbes loosely plus tightly attached to the solid matrix. 3. The activities of urease (EC 18.104.22.168), carboxymethylcellulase, glutamate dehydrogenase (EC 22.214.171.124) and alanine aminotransferase (glutamic-pyruvic transaminase) (EC 126.96.36.199) were significantly lower in the tannin-rich group, whereas the activities of glutamate ammonia ligase (glutamine synthetase) (EC 188.8.131.52; both γ-glutamyltransferase (EC 184.108.40.206) and the forward reaction) were higher in the tannin-rich group. These changes were more marked in micro-organisms tightly bound to the solid matrix than in the more complex microbial compartment. 4. The protein, DNA and RNA contents, and protein: RNA ratio, were significantly lower in the tannin-rich group, whereas no difference was observed for protein: DNA between the groups. 5. Effects of tannin-containing extracts of oak leaves on various rumen enzymes in vitro showed a trend similar to that observed in nylon-gauze bags, suggesting that the changes observed in various compartments were due to the tannins of oak leaves.