Haloperidol bound with equal affinity to σ and dopamine D-2 receptors (K I =2.8 nM). Compared to haloperidol, its carbonyl-reduced metabolite bound to σ receptors with nearly equal affinity. However, reduced haloperidol bound to dopamine receptors with 85-fold lower affinity compared to haloperidol (K 1=239 nM). The chlorophenyl-hydroxypiperidine metabolite of haloperidol lacked affinity for dopamine receptors, but bound with moderate affinity to σ receptors (K I =326 nM). The carboxylic acid metabolite lacked affinity for either receptor. Like haloperidol, (+)-pentazocine, and 1,3-di-o-tolylguanidine, reduced haloperidol potently inhibited the phosphoinositide response to muscarinic agonists in rat brain synaptoneurosomes, an assay which monitors σ agonist activity. This metabolite also produced a dystonic alteration of head position in rats when microinjected into the red nucleus. However, unlike observations with haloperidol and other σ ligands, this effect was associated with pathological changes in the red nucleus. Therefore, it cannot be attributed to σ receptor interactions with certainty. These findings suggest that administration of haloperidol results initially in effects mediated through both dopamine and σ receptors, but as metabolism proceeds the σ actions would be expected to decline at a significantly slower rate than the dopiminergic actions.