This study was designed to compare the acute response of mixed muscle protein synthesis (MPS) to rapidly (ie, whey hydrolysate and soy) and slowly (ie, micellar casein) digested proteins both at rest and after resistance exercise. Three groups of healthy young men (n= 6 per group) performed a bout of unilateral leg resistance exercise followed by the consumption of a drink containing an equivalent content of essential amino acids (10 g) as either whey hydrolysate, micellar casein, or soy protein isolate. Mixed MPS was determined by a primed constant infusion of l-[ring-13 C 6] phenylalanine. Ingestion of whey protein resulted in a larger increase in blood essential amino acid, branched-chain amino acid, and leucine concentrations than either casein or soy (P< 0.05). Mixed MPS at rest (determined in the nonexercised leg) was higher with ingestion of faster proteins (whey= 0.091±0.015, soy= 0.078±0.014, casein= 0.047±0.008%/h); MPS after consumption of whey was∼ 93% greater than casein (P< 0.01) and∼ 18% greater than soy (P= 0.067). A similar result was observed after exercise (whey> soy> casein); MPS following whey consumption was∼ 122% greater than casein (P< 0.01) and 31% greater than soy (P< 0.05). MPS was also greater with soy consumption at rest (64%) and following resistance exercise (69%) compared with casein (both P< 0.01). We conclude that the feeding-induced simulation of MPS in young men is greater after whey hydrolysate or soy protein consumption than casein both at rest and after resistance exercise; moreover, despite both being fast proteins, whey hydrolysate stimulated MPS to a greater degree than soy after resistance exercise. These differences may be related to how quickly the proteins are digested (ie, fast vs. slow) or possibly to small differences in leucine content of each protein.