Specimens of peritoneum from normal rat, mouse, man, uraemic patients and patients on chronic ambulatory peritoneal dialysis (C.A.P.D.) were examined by en face silver stained preparations and by electron microscopy. The ultrastructural features of the mesothelium and subjacent tissues in normal man, animal and uraemic patients were closely similar. Cellular degeneration and oedema were observed in the mesothelium of patients on C.A.P.D.