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The intestinal enterocytes provide the initial site for cytochrome P450 (CYP)-mediated metabolism of orally absorbed xenobiotics. In man and some animal species, the CYP3A subfamily is highly expressed in the intestines and considered to be important in the first-pass metabolism of drugs and other xenobiotics. The aim of the present study was to investigate the mRNA expression, immunohistochemical localization and catalytic activity of CYP3A in the intestines of horse. Real-time RT-PCR analyses showed that the highest CYP3A mRNA expression was present in the duodenum with a decreasing level towards jejunum, ileum, cecum, and colon. The CYP3A mRNA expression in the liver was similar as in the anterior part of the jejunum, but about 4.5 times lower than in the anterior part of the duodenum. Immunohistochemistry showed CYP3A immunoreactivity in the cytoplasm of the enterocytes, which decreased distally along the intestinal tract. CYP3A-dependent metabolic activity rose slightly from the anterior to the distal part of the duodenum and the anterior part of the jejunum and then declined to the middle and distal parts of the jejunum and the ileum, cecum, and colon. Our results suggest that CYP3A in the small intestine plays a major role in first-pass metabolism and may affect bioavailability and therapeutic efficiency of some orally administrated drugs in horse.