The aim of the present work was to study the influence of tannic acid (TA) and protein level in the diet on fermentation in the caecum of rats, activity of bacterial β-glucuronidase, and colon morphology. Twelve groups of six male Wistar rats were given either a control diet free of TA or diets containing 0.25, 0.5, 1, 1.5, or 2% TA. Diets contained 10 or 18% of crude protein. Body weight and feed intake were monitored during a 3-week experimental period. Tannic acid reduced protein apparent digestibility and decreased liveweight gain. Both factors affected caecal fermentation and increased volatile fatty acid production. The higher protein level increased the concentration of branched-chain fatty acids. Tannic acid reduced the activity of β-glucuronidase and affected colonic myenteron thickness. These results indicate that TA may have advantageous effects on the gastrointestinal tract if its amount in the diet does not exceed 1.5%.