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