1. Binding sites for the R and S enantianers of the 5HI2 agonistDDI (2,5_dimethoxy-4-iodophenylisopropylamim) were identified in rat brain using quantitative in-vitro autoradiography and compared with [125-I]-LSD binding. 2. In most regions of the brain,binding densit of the less active isomer 125-I-LSD was 15 to 85% of that exhibited by the active [ 135 I]R-DOI isomer. 3. Cortical menbrane preparations exhibited two binding sites,of the enantiomers with high (KdH) and low (KdL) affinity constants of 1.2 ± 0.02 nM and 29 ± 7 nM for the [125I)R-DOI and 2.1 ± 0.2 m and 18 f±4 nM for (125I)S~DOI, respectively.The respective high (BmaxH)and low (BmaxL) binding densities were 92 ±10 and 536 ± 164 fmol/mg protein for the [125I)R-DOI and 67 ± 19 and 245 ± 60 fmol/mg protein for (l25I)S-DOI. 4. Our results correlate with regional distribution of 5HT2 receptors reported in previous studies and indicate that DOI and its congeners have potential clinical applications for the in-vivo localization of 5HT2 receptors. hallucinogens, SEW.2receptors, lysergic acid diethylamide, phenylisopropylanines, psychotominetic, receptor autoradiography Abbreviations:bovine serum albmin (BSA); 4-bromo-2,5-dimethoxyphenylisopropylamine (DOB); 2,5-dimethoxy-4-iodophenylisopropylamine(DOI); lysergic acid diethylamide(LSD).