This study was undertaken to examine further the pharmacology of [3H]ifenprodil binding in rat brain at 37 degrees C. [3H]Ifenprodil bound specifically to membranes (Kd = 5.09 +/- 0.30 nM; Bmax = 2.36 +/- 0.19 pmol/mg protein). [3H]Ifenprodil binding was potently inhibited by sigma ligands and inhibitors of cytochrome P-450. The levorotatory enantiomers of pentazocine and SKF 10,047 were more potent inhibitors than corresponding dextrorotatory enantiomers. Furthermore, the pharmacological profile of [3H]ifenprodil binding was highly correlated with that of sigma 2 sites, not sigma 1 sites. The results suggest that [3H]ifenprodil labels sigma 2 sites in rat brain at 37 degrees C, and that [3H]ifenprodil would be useful for studying sigma receptor subtypes.