Chronic administration of the benzodiazepine, clonazepam, increased the number of [3H]5-hydroxytryptamine (5-HT1) binding sites in the frontal cortex of the rat. The increase reflected a change in the maximum density of binding sites (Bmax) with no change in ligand affinity (Kd). Increased binding occurred after continued exposure (10 days) to large (5.0 mg/kg) doses of clonazepam. The changes in [3H]5-HT binding were regional in that they occurred in membranes from the frontal cortex but not the brainstem. The effects were also at least partially selective for 5-HT receptors since the binding of the beta-adrenergic radioligand, [3H]dihydroalprenolol, was not affected by clonazepam. A second benzodiazepine, diazepam, did not affect the binding of [3H]5-HT at doses of 30 mg/kg per day. The latter data suggest that the effects of benzodiazepines on serotonin 5-HT1 receptors are unique to clonazepam.