Objective To assess both peritoneal insulin absorption and adsorption of insulin to the plastic delivery system, and to assess whether variation in peritoneal absorption is related to transport status. Design Eight insulin-dependent diabetic patients on continuous ambulatory peritoneal dialysis were studied. Insulin was added to the dialysate. The amount adsorbed by the bag tubing was determined; peritoneal absorption was measured by complete spent dialysate collection. Results Only 14% ± 5% of added insulin was adsorbed onto the dialysate delivery system. Marked variation in peritoneal absorption occurred (38% ± 14%, range 17% – 66%). Variations in membrane transport status did not account for variation in absorption. Conclusion Marked variation in peritoneal absorption of insulin occurs, accounting for some of the variation in intraperitoneal insulin requirements. This is not related to membrane transport status.