Evaluation of permeance in peritoneal dialysis. Relations were developed to calculate permeance based on a minimum number of experimental measurements. Permeance is the product of the effective peritoneal area times its mass transfer coefficient. The relations were used on clinical data obtained with and without nitroprusside, a direct vasodilator, added to the dialysate. Nitroprusside increased permeance 34% for BUN and about 100% for creatinine and inulin. The equivalent increase in clearance in a 50 minute exchange protocol would be 24, 60, and 83%. Normalized consecutive concentration data obtained every ten minutes in several long exchanges agree well with calculated values supporting the adequacy of the approach. An expression is also given to calculate the residual dialysate volume left from the previous dialysate exchange. The values calculated ranged from 200 to 500 ml.