[3H]serotonin binds to two classes of sites in a crude membrane preparation of Drosophila melanogaster heads, with apparent Kd values of 1.4 and 130 nM, and a concentration of 0.2 and 1.5 pmol/mg protein, respectively. High salt concentration and a guanyl nucleotide decrease the number of the high-affinity binding sites. Low concentrations of ergot alkaloids, various tryptamine derivatives, neuroleptic drugs, but not of phentholamine , propranolol and dopamine, displace [3H]serotonin from its high affinity sites. The relevance of the [3H]serotonin-binding sites to serotonin receptors in Drosophila is discussed.