Neutral and cationic indolyl radicals, such as those arising from tryptophan and melatonin, are involved in a variety of physiological situations. Due to their short life times, experimental characterization of these species is incomplete. We have performed density functional theory (DFT) calculations in order to provide information of the electronic spectral properties of indole, melatonin and tryptophan radical and radical cations. We predict that the neutral and cationic radicals exhibit absorption ranges from 450 to 500 nm and from 520 to 570 nm, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.