The in-vitro inhibition constants (Ki) of nine structural analogues of the potent 5-hydroxytryptamine (5-HT)-uptake inhibitor, 6-nitroquipazine, were determined to assess the structure-affinity relationship of these derivatives. The goal of these studies was to determine those positions on 6-nitroquipazine that could be derivatized without significantly decreasing the affinity of the drug for the binding site, so that radiolabels such as 123I, 76Br or 18F might be appended for in-vivo imaging studies of the 5-HT reuptake system. Using bromine as a steric probe, the rank order of potency of bromine-substituted 6-nitroquipazine analogues for inhibiting the binding of [3H]paroxetine to the 5-HT reuptake binding site was: 8- < 3- < 7- < 4- < 5-bromo. The in-vitro equipotent molar ratio (EPMR, Ki (analogue)/Ki(6-nitroquipazine)) of the 5-bromo analogue was 0.57, indicating that this analogue had greater affinity for the 5-HT reuptake complex than 6-nitroquipazine. Derivatization at the 5-position with fluorine and iodine also resulted in potent compounds with EPMR values of 1.1 and 0.83, respectively. Substitution of quipazine with bromo, cyano, and formyl groups at the 6-position produced less potent compounds than the 6-nitro group. Based upon the high affinities of the 5-bromo-, 5-fluoro- and 5-iodo-6-nitroquipazines for the 5-HT reuptake complex, these compounds are candidates for radiolabeling for in-vivo studies of the 5-HT reuptake site.