1. We have investigated glutamine transport in the perfused rat hindlimb using the paired-tracer isotope dilution technique. 2. Uptake of L-glutamine was stereospecific, saturable, sodium dependent, insulin sensitive and pH insensitive in the physiological range. The maximum capacity of transport (Vmax) under normal perfusate conditions at 37 degrees C, 145 mM-Na+ and in the absence of insulin was 1156 +/- 193 nmol min-1 g-1 with transport being half-maximal at a perfusate glutamine concentration of 9.25 +/- 1.15 mM. 3. The kinetics of Na+ dependence strongly suggested co-transport of Na+ and glutamine with a stoichiometry of 1:1; furthermore, Na+ activated the carrier without any change in the concentration of glutamine at which transport was half-maximal, i.e. a 'Vmax effect' rather than a 'Km effect'. 4. The characteristics of glutamine transport, especially its substrate specificity and the pattern of competitive and non-competitive inhibition of glutamine transport by other amino acids, suggest that it is mediated by a carrier or carriers for which asparagine and histidine are also suitable substrates. 5. The characteristics of muscle glutamine transport are related but distinct from those of system N identified in hepatocytes; we suggest that they are sufficiently distinct to justify the identification of a new variant of mammalian amino acid transport systems which may be identified by the symbol Nm. 6. The kinetic characteristics of system Nm are such that glutamine is likely to be the most rapidly exchanging amino acid across the muscle membrane at physiological intra- and extracellular glutamine concentrations. Its hormone and ion sensitivities are likely to be important in the physiological modulation of whole-body glutamine metabolism and also during derangements observed in disease and after injury.