The influence of adult age and adequacy of dietary protein intake on whole body glycine metabolism was studied in human subjects. Five healthy young adult males (19-25 yr) and six elderly males (64-78 yr) were given an adequate-protein diet (1.5 g protein/kg/day) for 7 days and a low-protein diet (0.4 g protein/kg/day) for 14 days. At the end of each dietary period, whole body glycine flux and rates of glycine synthesis were estimated with the use of a continuous 60 hr oral administration of 15N-glycine and determination of 15N enrichment of plasma glycine by gas chromatography-mass spectrometry with selected ion monitoring. Mean whole body glycine flux and the rate of endogenous glycine synthesis were 458 and 351 micromole/kg body weight/hr, respectively, for young adults receiving the diet adequate in protein; similar values were obtained in the elderly group. Feeding the diet low in protein resulted in an extensive and significant reduction in both parameters in young adults and also in elderly subjects to a similar extent. Measurement of 15N enrichment in plasma serine gave a constant ratio of 15N enrichment in plasma free serine relative to glycine for both age groups and at the two protein intake levels. It is concluded that aging of adults has little impact on the quantitative aspects of whole body glycine metabolism, but that it responds extensively to changes in protein intake. Thus, it appears that glycine synthesis and flux are integrated with the body's total nitrogen metabolism and requirement for dietary nitrogen.