The effect of dosing guinea pigs with spironolactone (100 mg/kg twice daily for 3 days) upon the sigma recognition site labelled with [3H]-DTG was investigated. Animals were dosed with 100 mg/kg spironolactone twice a day for 3 days. Spironolactone treatment caused a decrease in sigma radioligand binding in membranes prepared from liver and brain but not in adrenals or testes. Saturation analysis of [3H]-DTG radioligand binding in the brain indicated that the decrease in specifically bound [3H]-DTG binding was due to a reduction in receptor number with no change in affinity. This method for the selective depletion of brain and liver sigma recognition sites will provide a useful tool for exploring the functional significance of this site.