The purpose of this study was to evaluate the effect of patient position on the mass transfer area coefficient (KoA) and to characterize drain/fill profiles in an effort to enhance efficiency of automated peritoneal dialysis. Over 100 exchanges were performed in 38 stable peritoneal dialysis patients to either determine the small solute KoA in the supine versus upright position or to characterize fill/drain profiles. The KoA for all solutes tested was significantly greater in the supine position compared with the upright position (P < 0.05). Fill profiles revealed the fill rate to be a function of fill height (P < 0.001) and patient position (supine > upright [P < 0.001]). Analysis of drain flow rate versus time revealed an initial segment of high outflow (350 +/- 89 mL/min) followed by an abrupt transition to a segment characterized by slow drainage (36 +/- 21 mL/min). The first segment of drain only took 5.6 +/- 2.3 minutes (42% of the total drain time); in that time, 83% +/- 10% of the dialysate was drained. The transition volume (volume of dialysate remaining at the time the transition occurs, excluding residual volume) correlated with body surface area (R = 0.52, P < 0.01). In conclusion, automated peritoneal dialysis treatment (including intermittent peritoneal dialysis, which may be done in the upright position) should be done in the supine position to optimize the KoA, and shortening drain time to include only the initial segment of high outflow will improve the efficiency and convenience of therapy.