Patients undergoing peritoneal dialysis were studied to determine if peritoneal absorption was selective. Dialysis was performed using dialysate exchange schedules similar to those for intermittent peritoneal dialysis and continuous ambulatory peritoneal dialysis. The clearance rate from the peritoneal cavity during hourly dialysate exchanges was 6.2 ml/min for D (-)-lactate and 8.7 ml/min for L(+)-lactate (p less than 0.01). L(+)-Lactate disappeared more rapidly from the dialysate during the long-cycle exchanges. Our results suggest that clearance of lactate from the peritoneal cavity is relatively stereospecific and raises the question of selective absorption for other organic anions.