Radioligand binding studies were undertaken in the turtle whole brain, cerebellum and raphe using the selective radioligands [ 3 H]5-hydroxytryptamine trifluoroacetate ([ 3 ]5-HT) and [ 3 H]±8-hydroxy-2-(di-N-propylamino) tetralin hydrobromide ([ 3 H]DPAT) to identify serotonin (5-HT) receptors and the specific 5-HT 1 A receptor subtype. Scatchard analysis identified a nanomolar affinity binding site for [ 3 H]5-HT (12 nM) in turtle whole brain assays. A low affinity 5-HT 1 A site (102 nM) was also identified in turtle whole brain assays, with a higher affinity site noted in binding studies performed with tissue from the inferior raphe (20 nM). The difference in affinity for 5-HT receptors in reptilian versus mammalian brain may prove characteristic of lower vertebrate brains with implications for the physiologic effects of this neurotransmitter in the central nervous system.