We sought to determine whether ingestion of a between-meal supplement containing 30 g of carbohydrate and 15 g of essential amino acids (CAA) altered the metabolic response to a nutritionally mixed meal in healthy, recreationally active male volunteers. A control group (CON; n = 6, 38 ± 8 yr, 86 ± 10 kg, 179 ± 3 cm) received a liquid mixed meal [protein, 23.4 ± 1.0 g (essential amino acids, 14.7 ± 0.7 g); carbohydrate, 126.6 ± 4.0 g; fat, 30.3 ± 2.8 g] every 5 h (0830, 1330, 1830). The experimental group (SUP; n = 7, 36 ± 10 yr, 87 ± 12 kg, 180 ± 3 cm) consumed the same meals but, in addition, were given CAA supplements (1100, 1600, 2100). Net phenylalanine balance (NB) and fractional synthetic rate (FSR) were calculated during a 16-h primed constant infusion of l-[ring-²H₅]phenylalanine. Ingestion of a combination of CAA supplements and meals resulted in a greater mixed muscle FSR than ingestion of the meals alone (SUP, 0.099 ± 0.008; CON, 0.076 ± 0.005%/h; P < 0.05). Both groups experienced an improvement in NB after the morning (SUP, −2.2 ± 3.3; CON, −1.5 ± 3.5 nmol·min⁻¹·100 ml leg volume⁻¹) and evening meals (SUP, −9.7 ± 4.3; CON, −6.7 ± 4.1 nmol·min⁻¹·100 ml leg volume⁻¹). NB after CAA ingestion was significantly greater than after the meals, with values of 40.2 ± 8.5 nmol·min⁻¹·100 ml leg volume⁻¹. These data indicate that CAA supplementation produces a greater anabolic effect than ingestion of intact protein but does not interfere with the normal metabolic response to a meal.