The aim of this study was to investigate the effect of amino acid (AA) supplementation on the loss and its time course of free AA, during hemodialysis. 2 uremic patients on regular hemodialysis received an increasing number of essential AA tablets during the dialysis. The loss and its time course of 6 essential and 7 nonessential free AA during dialysis was assayed in the dialysate. The loss per dialysis treatment of the administered essential AA increased almost proportionally with the doses of AA tablets. For the nonessential AA, no definite relationship between doses of AA tablets and losses was observed. At all levels of supplementation, methionine was retained best: 90% of the amount present in the tablets. Threonine showed the lowest retention: 15-55%.