In eight, CAPD patients who either had insufficient results of dialysis treatment (six) or loss of ultrafiltration (two) on a normal scheme (4 X 2-1), the effects of a 3-1 dialysate exchange on the in situ intraperitoneal volume, solute mass transfer, and mass transfer area coefficients were compared with a 2-1 exchange. The solutes investigated were urea, lactate, creatinine, glucose, kanamycin, inulin, beta 2-microglobulin, albumin and IgG. The time course of the intraperitoneal volume showed marked interindividual variations. However, delta mean volume was lower with the 3-1 exchanges than with 2-1. This was the result of an increased water reabsorption rate during 3-1 (2.18 ml/min vs 0.94 ml/min, P less than 0.01), probably caused by increased lymphatic absorption. Peritoneal mass transfer of low-molecular-weight solutes (up to 500) was increased with the 3-1 exchange, but there was no evidence of any alteration in effective peritoneal surface area or peritoneal permeability. By comparing the mass transfer area coefficients of the various solutes to their free diffusion coefficients, it appeared likely that the diffusion of the low-molecular-weight solutes was unrestricted by membrane permeability and only hampered by the effective peritoneal surface area. For the proteins a size-selective restricted diffusion was found.