The effect of arginine and glycine supplementation on reducing body protein losses and on enhancing wound healing after trauma was studied in two age groups. Mature (4 month) and aged (24 month) Fischer 344 male rats were fed a diet containing 25% casein and 0.4% methionine with or without supplementation with 2.4% arginine . HCl and 1.0% glycine for 7 days before and after laparotomy. Nitrogen (N) balance studies (N intake - urinary N) were carried out during the last three pretrauma days and seven posttrauma days. The supplemented rats retained significantly more N than the controls and the mature rats significantly more than the aged rats. Polyvinyl alcohol sponges, implanted during surgery and removed from the rats on day 3 or 7 after surgery, were analyzed for hydroxyproline content and for the ratios of type III/type I collagen synthesized. Sponges obtained from the supplemented and the mature rats had more hydroxyproline and higher ratios of type III/type I collagen than those from the control and the aged rats. The beneficial effect of arginine and glycine supplementation on improving N retention in traumatized rats appears to be due, at least in part, to increased collagen synthesis in wounds.