The effects of inhibiting gamma-aminobutyric acid-transaminase (GABA-T) with aminooxyacetic acid or with gabaculine and inhibiting GABA uptake with nipecotic acid on dopamine (DA) synthesis in the retina of light-exposed rats were studied. 3,4-Dihydroxyphenylalanine (Dopa) accumulation after the inhibition of L-aromatic amino acid decarboxylase with NSD-1015 was used as an index of DA synthesis. The GABA-T inhibitors significantly increased GABA levels in the retina but had no effect on retinal Dopa accumulation in the light. Dopa accumulation was significantly inhibited at a high dosage by nipecotic acid alone and by low dosages of nipecotic acid in rats pretreated with either aminooxyacetic acid or gabaculine. The dosages of nipecotic acid which inhibited Dopa accumulation in the light also inhibited [3H]GABA uptake in the retina. The inhibitory effects of gabaculine and nipecotic acid on Dopa accumulation appeared to occur at least partially via GABA receptors because their action was significantly reversed by the GABA antagonist bicuculline methiodide. These experiments thus suggest that endogenous GABA can suppress the light-evoked increase in DA synthesis in the rat retina and support previous studies which concluded that an inhibitory GABA input may at least partially regulate the activity of the DA neurons in the retina of rats.