Selected neutral L-alpha-amino acids, and their dipeptides, were reversible, stereospecific, potentiators of GABA(B) receptor-mediated hyperpolarizing responses to baclofen (3-100 microM) in rat neocortical slices. These responses were sensitive to the GABA(B) receptor antagonist (+)-(S)-5,5-dimethylmorpholinyl-2-acetic acid (Sch50911) (30 microM). Most potent were L-Leu, L-Ile and L-Phe, as were the dipeptides L-Phe-Phe and L-Phe-Leu, and less potent were L-Met, L-Val, L-Cys, L-Cystine, L-Tyr, L-Thr, L-Arg and L-Ser. Inactive were L-Trp, L-His, L-Lys and L-Pro. These potentiators gave leftward shifts of the baclofen concentration-response curves with a Hill slope of 2, and a marked increase in the maximal hyperpolarizing responses. Selected L-amino acids and dipeptides are a class of naturally occurring GABA(B) potentiators, which may be allosteric modulators.