—The effects of a variety of acyclic or heterocyclic GABA analogues on GABA receptor binding and on high affinity transport of GABA in cultured astrocytes and mini-slices of brain cortex were studied. The receptor and transport sites were found to be stereospecific and they exhibited opposite stereoselectivity for (R)- and (S)-trans-4-amino-4-methylcrotonic acid and (R)- and (S)-β-proline. The most potent inhibitors of GABA binding were (RS)-4, 5-dihydromuscimol, muscimol, GABA, isoguvacine and isonipecotic acid with IC₅₀values of, respectively, 0.009, 0.006, 0.033, 0.037 and 0.33 μM. Under the present experimental conditions the following compounds inhibited preferentially the glial transport system: (3RS, 4SR)-4-hydroxynipecotic acid, guvacine, (RS)-N-methylnipecotic acid, (RS)-β-proline and β-alanine (IC₅₀ values 10, 25, 70, 320 and 1000 μM, respectively vs. 200, 100, 300, 1200 and >5000 for neuronal transport). On the other hand, (R)-trans-4-amino-4-methylcrotonic acid, (3RS, 4SR, 5SR)-4-hydroxy-5-methymipecotic acid and (RS)-3-hydroxy-5-aminovaleric acid preferentially inhibited neuronal transport as studied in mini-slices of brain cortex (IC₅₀ values 160, 300 and 430 μM, respectively vs. 500, > 5000 and 1400 μM for glial transport).