Radioligands are powerful tools for examining the pharmacological profiles of chemical leads and thus facilitate drug discovery. In this study, we identified and characterized 3-([1,1,1-(3)H]methyl)-2-(4-{[3-(1-pyrrolidinyl)propyl]oxy} phenyl)-4(3H)-quinazolinone ([(3)H]1) as a potent and selective radioligand for histamine H(3) receptors. Radioligand [(3)H]1 exhibited appreciable specific signal in brain slices prepared from wild-type mice but not from histamine H(3) receptor-deficient mice, demonstrating the specificity and utility of [(3)H]1 as a selective histamine H(3) receptor radioligand for ex-vivo receptor occupancy assays.