Melatonin binding sites were identified over the leptomeninges surrounding the human fetal brain using quantitative in vitro autoradiography and the melatonin agonist, 2-[ 1 2 5 I]iodomelatonin. Binding was found to be saturable and of high affinity (dissociation constant (K d )=54 pM and maximal theoretical binding (B m a x )=13 fmol/mg protein), and inhibited by guanosine-5'-o-(3-thiotriphosphate) (GTPγS) suggesting that these binding sites represent G protein-coupled melatonin receptors. RT-PCR performed on mRNA isolated from the human fetal leptomeninges detected expression of the G protein-coupled melatonin receptor Mel1a, but not Mel1b. In situ hybridisation confirmed the localisation of Mel1a mRNA transcripts over the leptomeninges of the fetal brain. The identification of 2-[ 1 2 5 I]iodomelatonin and Mel1a melatonin receptor expression in the fetal leptomeninges implies that melatonin may play a role in the early growth and development of the human brain.