An increasing amount of evidence suggests that the sigma (σ) sites, putative targets for a variety of psychotomimetic and antipsychotic drugs, exist not only in the brain but also in various peripheral organs. However, there are many ambiguities as to their biological roles, subcellular distributions, endogenous ligands and so on. We therefore performed our study for clarification of some of these ambiguities. As a result, we demonstrated that adult male rat liver microsomes, especially smooth endoplasmic reticulum, possessed a saturable haloperidol-binding site closely resembling the σ site, with a high affinity (Kd 1.0 ± 0.3 nmol/L) and high capacity (Bmax 9.3 ± 1.5 pmol/mg protein) and with the rank order of affinity of the ligands: haloperidol > reduced haloperidol > clorgyline > ifenprodil > 1,3-di(2-tolyl)guanidine, (—)-butaclamol > GBR-12909 > SKF-525A > progesterone > 5α-dihydrotestosterone > R(+)-3- (hydroxyphenyl)-N-propylpiperidine > testosterone ± corticosteroids, estradiol-17β, cholesterol and neuroactive compounds displaying high affinities for other neurotransmitter receptors such as dopamine D2, serotonin (5-HT1A and 5-HT2) and α1-adrenergic and GABAA receptors. This rank order showed a high correlation (r = 0.908) with that of a large portion (˜85%) of specific progesterone-binding site (Kd 31.0 ± 3.5 nmol/L, Bmax 5.7 ± 0.2 pmol/mg protein) of the same source. Therefore, these two sites were suggested to be the same or closely related. Furthermore, we provide a strong suggestion that these sites neither are identical with some subforms of the microsomal cytochromes P-450 or other steroid/drug-metabolizing enzymes nor participate universally and directly in the progesterone-metabolizing processes. (Hepatology 1994;20:1271–1280).