Increasing evidence indicates that the GABAergic system in cerebellar and limbic structures is affected in autism. We extended our previous study that found reduced [³H]flunitrazepam-labeled benzodiazepine sites in the autistic hippocampus to determine whether this reduction was due to a decrease in binding site number (B _max) or altered affinity (K _d) to bind to the ligand. Quantitation of hippocampal lamina demonstrated a 20% reduction in B _max indicating a trend toward a decreased number of benzodiazepine binding sites in the autistic group but normal K _d values. A reduction in the number of hippocampal benzodiazepine binding sites suggests alterations in the modulation of GABA_A receptors in the presence of GABA in the autistic brain, possibly resulting in altered inhibitory functioning of hippocampal circuitry.