FAEFHI, also known as 4-(2-aminoethyl)-5-hydroxyindole, is a serotonin receptor modulator of the indole group related to serotonin and other tryptamines. It is a positional isomer of serotonin in which the ethylamine side chain at the 3 position of the indole ring system has been moved to the 4 position. FAEFHI can also be thought of as a greatly simplified analogue of LSD and hence partial ergoline. Although FAEFHI shows significant affinity for serotonin receptors, it had much lower affinity than serotonin or tryptamine (171-fold lower than serotonin and 5-fold lower than tryptamine). In addition, it did not show serotonin-like activity (i.e., serotonin receptor agonism) even at very high concentrations in vitro. FAEFHI was first described in the scientific literature by 1984.