There has been limited investigation as to whether a predominantly forage-based diet can provide adequate amounts of limiting amino acids (AA) to horses. The first objective was to determine if AA supplementation of AA believed to be limiting to protein synthesis in forage-based diets would affect measures of whole-body protein metabolism in sedentary mature horses. The effect of forage type (timothy or alfalfa) and AA supplementation (lysine, threonine or histidine) on plasma urea nitrogen (PUN) and AA concentrations and measures of whole-body phenylalanine kinetics were evaluated. There was no effect of either forage type or AA supplement on rates of whole-body protein synthesis (P > 0.05). The second objective was to determine the effects of either timothy or alfalfa hay supplemented with either a high or low protein ration balancer on measures of whole-body protein metabolism in yearling horses. The effect of forage type and the ration balancer protein level on concentrations of PUN, plasma AA and measures of wholebody phenylalanine kinetics were evaluated. There was no effect of treatment on average daily gain (P = 0.18). When horses consumed the alfalfa-based diets, rates of phenylalanine flux, oxidation and use for protein synthesis were greater than when they consumed timothy-based diets (P < 0.05). Phenylalanine use for protein synthesis was not affected by the protein level of the ration balancer (P = 0.3). Yearling horses achieve greater rates of protein synthesis when fed alfalfa-based diets, compared to timothy-based diets, supplemented with a low protein ration balancer.