A prospective two year follow-up study of the functional characteristics of the peritoneal membrane was conducted in 61 CAPD patients. Peritoneal transport of solutes, calculated by mass transfer area coefficients for urea and creatinine, peritoneal clearances for proteins, percentage of absorption of glucose, as well as net ultrafiltration were measured every four months. After five months on CAPD a decrease was found for the transport of most solutes (P < 0.05, mean values, ml/min/1.73 m2): urea 18.1 to 16.2, creatinine 9.5 to 8.4, IgG 0.049 to 0.040 and alpha 2-macroglobulin 0.020 to 0.015, as well as for the absorption of glucose (57.9 to 53.2%, P < 0.05). Net ultrafiltration increased simultaneously from 44.6 to 100.5 ml/4 hr/1.73 m2, P < 0.05. From five months to two years on CAPD a significant increase in the transport of all solutes except alpha 2-macroglobulin was found, as well as a decrease in net ultrafiltration. Peritoneal transport at the end of the study was not significantly different from the starting values. Our findings indicate an initial effect of CAPD itself on peritoneal transport, probably due to the recent start of the treatment. Baseline values were reached after five months on CAPD. Thereafter a gradual increase in peritoneal solute transport occurred during two years of treatment. This can be explained by an increase in the effective peritoneal surface area.