The morphological and biochemical development of fetal rat intestine was examined for up to 5 weeks following transplantation to syngeneic hosts at 17 and 20 days of gestation. In transplants of both ages, normal villi bearing mature enterocytes developed. In addition, the disaccharidases lactase, maltase, and sucrase, as well as alkaline phosphatase, underwent normal patterns of development. Lactase activity, initially high, fell significantly, while maltase and sucrase activities increased significantly in the interval between 2 and 5 weeks following transplantation. During this same period, alkaline phosphatase developed the proximally located, high-activity form. The transplanted intestine also developed normal topographical distributions of enzyme activities. Measurement of corticosterone levels demonstrated that, except for a transient upsurge at the time of operation, hormone levels did not change significantly during the period of transplant maturation. These data indicate that the brush-border enzymes of the small intestine develop according to an intrinsic program which is already established as early as 17 days of gestation.