Captive gray squirrels were fed acorns or diets containing commercial tannins to determine the effects of tannins on digestion and detoxification (glucuronidation) activity. In the Acorn experiment, Northern red oak acorns (Quercus rubra), white oak acorns (Q. alba), or rat chow were fed to squirrels. Levels of phenols, condensed tannins, and hydrolyzable tannins were higher in red oak acorns than white oak acorns and were likely responsible for the lower dry matter intake, lower apparent protein digestibility, lower digestible protein and energy intakes, and higher glucuronidation activity observed in squirrels fed red oak acorns. Although apparent protein digestibility and digestible protein intakes were reduced on a white oak acorn diet, this diet did not continuously suppress dry matter intake or stimulate glucuronidation. It appears that gray squirrels may not be able to subsist on red oak acorns alone, but may require other foods to dilute tannin intake and provide additional nutrients. In the Tannin experiment, squirrels were fed rat chow containing no tannins, 4% or 8% tannic acid (hydrolyzable tannin), or 3% or 6% quebracho (condensed tannin). Apparent protein and energy digestibilities of tannic acid-containing diets were lower than the control. These reductions were likely due to the formation of strong complexes between protein and high molecular weight gallotannins. These complexes may have simultaneously protected these gallotannins from hydrolysis and allowed tannic acid to reduce digestive efficiency. Apparent protein and energy digestibilities of quebracho-containing diets were reduced, indicating protein complexing by these nonhydrolyzable tannins. Consistent with the hypothesis that hydrolyzable tannins are more likely to be broken down and absorbed internally than condensed tannins, only the 8% tannic acid diet tended to increase glucuronide excretion. / Master of Science