Daily measurements of nitrogen balance were made at two levels of protein intake in five patients undergoing chronic intermittent dialysis therapy. During ingestion of high (1.4 g/kg of body wt) protein intake, nitrogen balance was positive on nondialysis days and negative on dialysis days, so that cumulative balance for the week of study was not different from zero. During ingestion of low (0.5 g/kg) protein intake, nitrogen balance was approximately zero on nondialysis days but was again negative on dialysis days, so that cumulative balance for this period was negative. The negative nitrogen balance observed on dialysis days was associated with a higher rate of urea nitrogen generation (Gu, g/24 hr, determined by a kinetic model of urea nitrogen in dialysis patients) that was most evident in the hours immediately following dialysis. Net protein catabolic rate (PCR, g/24 hr), derived from total nitrogen mass balance equations, correlated very closely with Gu:Gu = 0.154 PCR - 1.7, r = 0.96. This relationship agreed well with previous observations made in nondialyzed uremic patients under more steady-state conditions. These studies demonstrate that nitrogen balance is negative on dialysis days regardless of protein intake, and that Gu is higher on dialysis days. The negative nitrogen balance could result from amino acid loss in dialysate and from increased protein catabolism stimulated by loss of glucose into dialysate.