The rates of nitrogen flux and protein synthesis in the whole body were measured in two intensive care (ICU) patients on two occasions separated by a period of 24 h. Rates of flux and synthesis were estimated from 15N excretion in urinary ammonia after a slow injection of 15N glycine over a period of 1 h. The aim of the present study was to evaluate the feasibility and the reproducibility of the method. The variation of nitrogen flux was between 6 and 9%, and for synthesis between 7 and 9%. Skeletal muscle protein breakdown was measured from urinary 3, methylhistidine excretion. The muscle contribution to the whole body breakdown rate was 32 and 41% for one patient and 48 and 51% for the other one. The combination of measurement of 15N excretion after a single dose of 15N glycine with that of urinary 3, methylhistidine provided a reproducible method for measuring whole body protein synthesis in ICU patients. It could be repeated at short intervals and gave useful comparative information provided that conditions were carefully standardized.