The uptake of L-carnitine was characterized in mouse brain synaptosomal preparations, with an emphasis on mutual interactions with GABA uptake systems. The uptake consisted of nonsaturable diffusion and one saturable energy- and sodium-dependent component. GABA, L-DABA and nipecotate were strong and hypotaurine and homotaurine moderate inhibitors of the uptake. The inhibition by GABA was shown to be competitive. GABA uptake contained two saturable transport components, high- and low-affinity. It was most strongly inhibited by nipecotate and L-DABA, but also by carnitine and hypotaurine. The high-affinity uptake of GABA was competitively inhibited by carnitine, but the inhibition of the low-affinity uptake of GABA was of the mixed type. The results suggest that GABA and carnitine share the same carrier system at synaptosomal membranes. However, GABA is the preferred substrate and the carnitine concentrations which significantly inhibited GABA uptake exceed the physiological carnitine levels in vivo.