Peritoneal transport studies provide information on the vascular site the peritoneal membrane. The mass transfer area coefficient of creatinine mainly reflects the vascular peritoneal surface area, while the restriction coefficient to macromolecules reveals the intrinsic permeability of the peritoneal membrane. The Na+ dialysate/plasma ratio after a one hour 3.86% glucose dwell can be used as a rough indication of aquaporin-mediated water transport. Dialysate concentrations of cancer antigen 125 reflect the mesothelial cell mass. Ultrafiltration failure is the most frequent transport abnormality in long-term peritoneal dialysis and peritoneal sclerosis. Its association with high mass transfer area coefficients of creatinine suggests an increase in the vascular surface area due to neoangiogenesis, but the aquaporin-mediated water transport may also be disturbed. The decrease in dialysate CA 125 is compatible with a loss of mesothelial cell mass in long-term peritoneal dialysis. A proposal for a peritoneal membrane test is put forward. The main differences with the peritoneal equilibration test are: the use of 3.86% glucose, the determination of the Na+ dialysate/plasma ratio after one hour and of the dialysate CA 125 concentration after four hours.