During muscle-protein wasting associated with injury and disease the distribution ratio of free glutamine between muscle and blood falls. In pursuing possible consequences of this, we investigated the relationship between the rate of muscle protein synthesis and intramuscular glutamine concentration, manipulated acutely in the isolated perfused rat hindquarter. Increasing perfusate glutamine from 0.67 to 5.0 mM caused a 200% increase in intracellular glutamine and a 66% increase in protein synthesis in the absence of insulin; in the presence of insulin a 30% increase in intramuscular glutamine was accompanied by an 80% increase in protein synthesis. Analysis of variance of the results confirmed the existence of positive relationships between intramuscular glutamine and protein synthesis in the presence or absence of insulin. Control of the size of the intramuscular free pool of glutamine may be important in determining the muscle protein mass.