The present study evaluates the influence of the preceding exchange on peritoneal equilibration test (PET) results in patients on automated peritoneal dialysis (APD). A standard PET was performed following a 24-hour CAPD period with a preceding long overnight dwell of 8-10 hours (PETST), and following the usual APD regimen with short overnight dwell (PETAPD). We evaluated 9 patients of mean age 59 +/- 18 years. Mean time on peritoneal dialysis was 31 +/- 17 months, and mean APD duration was 15 +/- 11 months. Mean D/P creatinine at 4 hours was 0.77 +/- 0.12 (PETST) and 0.77 +/- 0.13 (PETAPD, p = 0.901). Mean D/D0 glucose was 0.33 +/- 0.07 (PETST) and 0.36 +/- 0.09 (PETAPD, p = 0.347). A significant correlation was seen between the D/P creatinine ratios (r = 0.946, p < 0.001) for the two PET methods and the D/D0 glucose ratios (r = 0.554, p = 0.017) for the two PET methods. Transport classification did not change in any patient. The current data support the use of the PET for peritoneal membrane evaluation immediately after cycler therapy in APD patients. We suggest that there is no need to change the dialysis regimen to a long dwell for the preceding exchange to evaluate peritoneal membrane characteristics when D/P creatinine and D/D0 glucose ratios are measured.