Aspects of the cell kinetics of the developing intestinal epithelium of fetal sheep were investigated by autoradiography. The sites of (3H)-thymidine uptake were localized in the small intestinal epithelium of fetal sheep from 60 to 145 days' gestation. Labeled cells were always confined to the intervillus progenitive zone, either between the bases of adjacent villi in the younger fetuses or in crypts. There were no labeled cells within the villus epithelium. Labeled and unlabeled cells within the progenitive zones were counted in proximal and distal regions of the small intestine. The proportion of cells labeled (PCL) was unaffected by the route of administration of (3H)-thymidine into the fetus. There was no clear age-related trend in PCL of the distal region (mean distal PCL, 0.23 +/- 0.03). On the other hand, in proximal regions, PCL was at its highest value at 60 days (0.33), falling to a minimum of 0.14 at 130 days, before rising to 0.25 (+/- 0.04) at 136 days. These results demonstrate that the proliferation of enterocytes in utero is qualitatively similar to that in the adult, being confined to a distinct progenitive zone. Such a finding is an important first step in understanding changes in enterocyte structure and function, since this may, as seen in the adult, be largely determined by the rates of division and renewal.