Degenerate primers, suitable for polymerase chain reaction studies and based on the conserved structure of G protein-coupled receptors, were used to isolate cDNA clones encoding putative G protein-coupled receptors from a human hippocampal cDNA library. One clone isolated by this approach (AC1) encoded a putative receptor with 39% amino acid sequence identity to the serotonin 5HT1A receptor and 47% identity to the 5HT1D receptor. When expressed transiently in the human embryonic kidney cell line 293, AC1 cDNA-encoded receptor displayed high affinity (Kd = 15 nM) and saturability for [3H]serotonin, suggesting that AC1 encodes a 5HT1-like receptor. However, 5-carboxamidotryptamine demonstrated low affinity (pKi = 5.15) compared with serotonin (pKi = 8.14), consistent with the observed binding of the putative 5HT1E receptor. The excellent correlation observed between the pharmacology of the expressed receptor encoded by AC1 and the human brain 5HT1E binding site confirms that AC1 encodes a 5HT1E receptor and establishes a fifth 5HT1-like receptor subtype.