sigma Receptors have been the focus of extensive studies because of their potential functional role in several important physiological and biochemical processes. To further evaluate the properties of sigma receptors, especially sigma-1 and sigma-2 subtypes, we have synthesized a series of N,N'-disubstituted piperazine compounds (1-32). The design of these compounds was based upon the early structure-activity relationship (SAR) studies of the minimum structural requirements of a molecule necessary to elicit sigma receptor binding activity. In the N-(3-phenylpropyl)piperazine series, compounds with the ethylenediamine moiety (8-11, 15-17) showed 6-20-fold higher affinity for sigma-1 and 2-40-fold higher affinity for sigma-2 relative to their corresponding amides (1-7). The (m-nitrophenethyl)piperazine 10 exhibits a subnanomolar affinity for the sigma-1 site, whereas the corresponding o-nitro compound 9 shows the highest affinity for the sigma-2 site (Ki = 4.9 nM). Compounds with a free amino terminus were designed as precursors for use as bioconjugated affinity compounds. Some of these compounds displayed high affinity for sigma-1 and moderate affinity for sigma-2 sites and are currently used for the purification and characterization of the receptor subtypes.