Evidence suggesting a reduction of cerebral gamma-aminobutyric acid (GABA) neurons in Alzheimer's disease has been reported. To evaluate the possible contribution of GABA system dysfunction to the intellectual decline associated with this disorder, a controlled therapeutic trial of a potent and specific GABA agonist, THIP [4,5,6,7-tetrahydroisoxazolo(5,4,-c)pyridin-3-ol], was undertaken. Six Alzheimer patients with mild to moderately severe dementia and low spinal-fluid GABA levels received THIP at maximum individually tolerated dosage. No significant change in cognitive function could be discerned, despite attainment of dose levels that produced centrally mediated adverse effects similar to those of other GABA agonists. The results support the views that pharmacologic attempts to stimulate central GABA-mediated synaptic function may not confer therapeutic benefit to patients with Alzheimer's disease and that a GABA system deficit may not serve as a critical determinant of the dementia that characterizes this disorder.