The effect of 15 different amine substituents on 5-HT2A and 5-HT2C serotonin receptor binding was investigated for two series of compounds (i.e., phenylalkylamine and indolylalkylamine derivatives). In general, amine substitution decreases receptor affinity; however, N-(4-bromobenzyl) substitution results in compounds that bind at 5-HT2A receptors with high affinity (Ki < 1 nM) and with > 100-fold selectivity. Although parallel structural modification in the two series result in parallel shifts in 5-HT2C binding, these same modifications alter 5-HT2A binding in a less consistent manner.