The cellular location of 5-HT 1 A and 5-HT 1 B binding sites in the hippocampus was investigated using a stereotaxic unilateral lesion of the CA1 field by ibotenic acid, followed by autoradiography on brain sections with the specific ligands [ 3 H]8-OH-DPAT and S-CM-G[ 1 2 5 I]TNH 2 , respectively. As compared to the contralateral side of the lesion, the ipsilateral side exhibited a decrease in the density of 5-HT 1 A binding sites in the strata oriens and radiatum of CA1, and of 5-HT 1 B binding sites in the dorsal subiculum and stratum oriens of CA1. The results demonstrate that 5-HT 1 A binding sites are located on dendrites whereas 5-HT 1 B binding sites are located on axon terminals of CA1 pyramidal cells.