The aims of this study were to determine whether glutamine infusion prevents the decline in protein synthesis and muscle wasting associated with repeated glucocorticoid treatment. Hormone (cortisol acetate, 100 mg.kg body wt-1.day-1) and vehicle (carboxymethyl cellulose)-treated female rats were infused with either saline or glutamine (240 mM, 0.75 ml/h) for a 7-day period. Glutamine infusion attenuated the decline of plantaris muscle glutamine concentration (3.0 +/- 0.2 vs. 2.3 +/- 0.2 mumol/g) and prevented > 70% of the total muscle mass losses due to the glucocorticoid injections. Fractional synthesis rates of myosin heavy chain (MHC) and total protein were determined after constant [3H]leucine infusion from the leucyl-tRNA precursor pool, which was similar in all groups (range 4.8 +/- 0.5 to 6.3 +/- 0.4 disintegrations.min-1.pmol-1). MHC synthesis rates (%/day) in plantaris muscles were reduced to approximately 40% of controls (4.2/9.4). Although glutamine had no effect on MHC synthesis in vehicle-treated animals (10.1/9.4), it prevented 50% (7.6/4.2) of the hormone-induced decline in MHC synthesis rates. The same results were obtained with total protein synthesis measurements. Changes in muscle mass did not appear related to estimates of protein breakdown. In conclusion, these data show that glutamine infusion is effective therapy in counteracting glucocorticoid-induced muscle atrophy. Atrophy attenuation appears related to maintaining muscle glutamine levels, which in turn may limit the glucocorticoid-mediated downregulation of MHC synthesis.