The sigma binding sites are postulated to be involved in various central nervous system (CNS) disorders. The neuroleptic drug, haloperidol, displays high affinity for these receptor sites in the CNS. In the present study the effect of repeated exposure of rats to haloperidol (4 mg/kg/day for 14 days) on sigma binding sites labeled with (+)-3-(3-hydroxyphenyl)-N-1-(propyl)piperidine [+)-3-PPP) and 1,3-di-o-tolyl-guanidine (DTG) was investigated. In addition, the regulatory effect of guanine nucleotides on the binding of these two ligands to brain membranes derived from saline and haloperidol-treated rats was examined. Repeated administration of haloperidol induced down-regulation of (+)[3H]-3-PPP binding sites (75% decrease in the number of binding sites compared to control) which persisted for at least 7 days after termination of the haloperidol-treatment. The down-regulation of (+)-3-PPP binding sites was accompanied by reduced responsiveness to guanine nucleotides (i.e. 5-guanylylimidodiphosphate (Gpp(NH)p) compared to the sensitivity of (+)-3-PPP binding sites to the nucleotides tested in control membranes. However, at the 28th day after termination of the haloperidol-treatment, a complete recovery in the total number of (+)[3H]-3-PPP binding sites was observed, and the sensitivity to guanine nucleotides was regained. These findings suggest a marked plasticity in (+)-3-PPP/sigma receptor binding activity. In contrast, [3H]DTG binding sites expressed neither sensitivity to the repeated exposure to haloperidol nor to guanine nucleotides, suggesting a distinction between DTG and (+)-3-PPP binding sites in rat brain.