Anephric rats were maintained on continuous ambulatory peritoneal dialysis (CAPD). Peritoneal permeability was assessed during a standard 4-hour peritoneal equilibration test (PET) performed with Dianeal 3.86% (Baxter Healthcare, Deerfield, Illinois, U.S.A.). The effect of uremia on peritoneal permeability was evaluated in an experimental protocol in which each animal served as its own control. In each rat, PET1 (control) was performed before removal of kidneys and PET2 (uremia) was performed four days after removal of kidneys. Net ultrafiltration during a 4-hour exchange with Dianeal 3.86% was higher during PET1 (3.8 +/- 2.3 mL) than during PET2 (-1.3 +/- 3.3 mL), p < 0.05. Peritoneal permeability to urea and glucose was similar in both series. Transperitoneal equilibration of creatinine concentration was faster in uremic animals: D/P at 4 hours was 0.94 +/- 0.06 during PET2 versus 0.77 +/- 0.08 during PET1, p < 0.001. The opposite difference was seen for total protein: D/Px 1000 after a 4-hour dwell was 51.4 +/- 19.8 during PET2 versus 70.3 +/- 12.9 during PET1, p < 0.05. Our results show that uremia modifies the permeability of the peritoneum to both water and solutes.