On the basis of the stucture of genistein, a new series of 3-arylquinazolines was prepared and tested for their estrogen receptor (ER) alpha and beta affinities. 5,7-Dihydroxy-3-(4-hydroxyphenyl)-4(3H)-quinazolinone (1aa) acts as an agonist on both ER subtypes. It has 62-fold higher binding affinity [IC(50)(ERbeta) = 179 nM] and 38-fold higher functional potency in a transcription assay [EC(50)(ERbeta) = 76 nM] with ERbeta than with ERalpha, thus improving upon the selectivity of genistein. All of the analogues showed preferential binding affinity for ERbeta. Many are also more potent in activating transcription by ERbeta than by ERalpha. Transformation of the C=O functionality at position 4 into a C=S group provided 5,7-dihydroxy-3-(4-hydroxyphenyl)-4(3H)-quinazolinethione (1ba), which acts as an agonist on both ER subtypes but has 56-fold higher binding affinity for ERbeta over ERalpha [IC(50)(ERbeta) = 47 nM] and 215-fold higher potency in the transcription assay [EC(50)(ERbeta) = 13 nM]. These ERbeta-selective compounds may represent valuable tools in understanding the differences in structure and biological function of ERbeta and ERalpha.