Abstract Whole urine samples, obtained from patients after a thermal burn, were hydrolyzed with 6 N HC1 at 100° for varying periods of time. There was no uniform relationship between the hydrolysis time and the recovery of any of the amino acids. For maximum recovery the hydrolysis times varied from 3 h, for aspartic acid, proline, and glutamic acid to 24 h for valine, isoleucine, leucine and phenylalanine. The maximum recoveries of the basic amino acids were achieved with hydrolysis times of 12 h or less. Determination of the basic amino acids was complicated by the formation of considerable ammonia, which accounted for nearly 50% of the nitrogen in the sample after 46 h hydrolysis. The observations indicate the diverse nature of the peptide components of urine, and delineate the difficulties involved in determining the total concentration of any of the amino acids.