Stimulated by the development of continuous ambulatory peritoneal dialysis (CAPD), it was only in the last decade that normal and dialysate-exposed peritoneum were examined by electron microscopy. Biopsies of parietal peritoneum from patients on CAPD show ultrastructural alterations in the mesothelium consisting of an increased development of the rough endoplasmic reticulum but a decrease in surface microvilli and micropinocytotic vesicles as compared to tissue from normal controls or uremic individuals. Alterations in amount and consistency of submesothelial ground substance, and number and disposition of collagen fibers combined with mesothelial changes together constitute the ‘reactive’ peritoneum of peritoneal dialysis.