alpha-Aminoisobutyric acid (AIB) transport into the intracellular compartment of extensor digitorum longus and soleus muscles was measured (in vitro) after allowance for the equilibration of the amino acid in the extracellular space. The latter was determined with three markers, [14C]inulin, 60Co-EDTA and [3H]mannitol. Net transport of AIB was subsequently divided into its two components, i.e. influx and efflux. Rates of influx were measured as the intracellular accumulation of [14C]AIB after a short incubation (5 min), and efflux was measured as the release of AIB with time (up to maximum of 50 min) from muscles that had previously been preloaded with AIB. This intracellular efflux was resolved into two phases, which probably represent two separate components of exit. The influence of extracellular Na+ on the transport of this neutral amino acid (representing the A system) was investigated. Na+ depletion resulted in lower accumulations of AIB, the effects becoming more pronounced with progressive depletions of external Na+. These changes arose from an inhibition of AIB influx, concomitant with an enhancement of its efflux. In contrast, all components of tyrosine transport (representing the L system) were unaffected by lowering external Na+ concentrations. The net accumulation of AIB was also suppressed by cortisol. This inhibitory effect was, however, Na+-dependent and resulted solely from the steroid's enhancement of AIB efflux, the hormone being without effect on AIB influx.