The histochemistry and DNA turnover of postcapillary high-endothelial venules in lymph nodes and Peyer's patches have been investigated. Autoradiographic studies showed that the cells are long lived. They exhibit a distinct metachromatic reaction that is indicative of a moderate amount of RNA. They are PAS positive due to the presence of glycogen. Considerable amounts of mucopolysaccharides are not found inside the cells. The reactions for lactate dehydrogenase, NADH-tetrazolium reductase and unspecific esterase are strong. The intensities of the reactions are comparable to those of the epithelium of the intestinal villus and of reticular cells and definitely stronger than that of other endothelial cells. Tests for acid phosphatase, alkaline phosphatase, succinic dehydrogenase, glucose-6-phosphate dehydrogenase and NADPH-tetrazolium reductase showed that these enzymes are either absent or present in small amounts.