Synthesis and structure-activity relationship studies of a series of 4-aminoquinazoline derivatives led to the identification of (1R,2S)-17, N-[(1R,2S)-2-({2-[(4-chlorophenyl)carbonyl]amino-6-methylquinazolin-4-yl}amino)cyclohexyl]guanidine dihydrochloride, as a highly potent ORL1 antagonist with up to 3000-fold selectivity over the mu, delta, and kappa opioid receptors. Molecular modeling clarified the structural factors contributing to the high affinity and selectivity of (1R,2S)-17.