To evaluate the effects on the GABAergic system, Wistar rats were raised on a chronically protein- and tryptophan-restricted diet with 8% protein, based on either Purina chow or corn. There was a significant decrease in both body and cerebral weight in the restricted animals compared with the control group fed with a 23% protein diet. In animals fed mainly corn, glutamic acid decarboxylase (GAD) activity increased significantly at the ages studied (14, 30, and 60 days) in the cerebral cortex and hippocampus. In the same way, gamma-aminobutyric acid (GABA) release decreased significantly in early life in both brain regions, then increased in 30-60-day-old animals corn-fed predominantly in the cerebral cortex. The reduction in GABA release may be attributable to a decrease in GABAergic cell density, which could induce an over-activation of 5-hydroxytryptamine (5-HTergic) receptors, leading in turn to the observed enhancement of GAD activity. Taken together, these results may represent a plastic response by GABAergic neurons to (5-HTergic under-stimulation in mainly corn-fed animals.