The activity of aspartate aminotransferase, alanine aminotransferase and lactate dehydrogenase, total protein, albumin and cholesterol levels were determined in the blood serum of Japanese quails infected with coccidia and treated with Baycox (active ingredient: toltrazuril). Lower levels of AST and ALT activity were noted in treated birds regardless of the applied Baycox dose. The biochemical changes observed in the blood serum of Japanese quails point to coccidia-induced damage of digestive system tissues despite an absence of pronounced clinical symptoms. Significantly lower levels of AST activity and higher levels of LDH activity in treated birds indicate that coccidiosis treatment with toltrazuril contributed to the regeneration of digestive system tissues. An insignificant increase in cholesterol levels was noted, whereas the other serum biochemical parameters remained within the reference ranges.