In most neonatal animals, the small intestinal epithelium is responsible for endogenous arginine production. The ability of neonatal enterocytes to synthesize arginine immediately after birth suggests that the enzymes involved are present prenatally. Pyrroline-5-carboxylate is the common intermediate in the intestinal pathways for the synthesis of citrulline and arginine from both glutamine and proline and is interconverted into ornithine by ornithine aminotransferase (OAT). In this study, OAT enzymatic activity and mRNA expression in the intestine of fetal pigs from 30 to 110 d of gestation were determined. Enzymatic activity (nanomoles per minute per milligram of protein) peaked at d 45 of gestation and increased again between d 60 and 110 of gestation. At 30 and 35 d of gestation, OAT mRNA expression was detected throughout the mucosal epithelium of the small intestine. Throughout the remainder of gestation, OAT expression was notably higher in the villus epithelium than in the crypt epithelium. The presence of OAT in the small intestinal epithelium throughout gestation suggests that the porcine small intestine is capable of interconverting ornithine and pyrroline-5-carboxylate during fetal development. This capability may be important for synthesis of arginine, proline, ornithine, and polyamines for development and metabolic activity of the intestine during gestation or for somatic growth of the fetus.