During the evaluation of 1% amino acid solution as an alternative osmotic agent to glucose, we measured amino acids and proteins in dialysate, urine and plasma to evaluate the uptake of amino acids and their effects on membrane permeability. Eight patients (plasma albumin less than 35 g/l) were on 21 exchanges of glucose fluid for 4 weeks before and after 12 weeks, during which a solution of 15 amino acids (Baxter '151') was used for the morning exchange. The absorption of amino acids from the single daily '151' exchange increased during the study: 16.4 g at 4 weeks and 17.1 g after 12 weeks (P less than 0.01) with increases in eight amino acids. Amino acid uptake was related to the permeability characteristics of the patients. Following each '151' exchange, 1% of the amino acids absorbed were dialysed into subsequent glucose exchanges. Consequently the net daily gain was 15.0 g increasing to 15.6 g, whereas daily depletion during glucose exchanges was 1.8 g both before and after '151'. Clearance of five proteins increased both at the start and after 12 weeks of '151'. Total protein and prealbumin loss into dialysate increased by about 20%, and when glucose was restored loss of transferrin, albumin and immunoglobulin G decreased. Urinary concentrations were similar throughout. Amino acid uptake from '151' greatly exceeded all losses although our results suggest small reversible increases in macromolecular permeability of the peritoneum.