Using homogenates of catfish whole-brain in an isotonic medium, we observed an accumulation of [3H]GABA that was temperature-sensitive and was dependent on the presence of sodium ions, the optimum concentration of which was 75 mM. A kinetic analysis showed that the [3H]GABA uptake mechanism became saturated with increasing GABA concentrations. A high-affinity system, only, was evident whose Km was calculated as 12 microM. Four structural analogues of GABA were found to be competitive inhibitors of uptake, and Ki values were determined. Nipecotic acid (Ki = 1.8 microM) and guvacine (Ki = 3.9 microM) were the most potent compounds, however 2,4-diaminobutyric acid (Ki = 8.9 microM) and beta-alanine (Ki = 55 microM) also had an effect. The characteristics of the uptake mechanism in catfish brain that we have studied are similar to those reported for uptake by mammalian brain except that in the latter, both a high- and a low-affinity transport processes are present. Our data, taken together with what is already known, strongly suggest that the biochemistry of the GABA system in lower vertebrates does not differ significantly from that in mammals.