SAR studies of lead GnRH receptor antagonists 2a and 2b reported earlier resulted in the discovery of compound 10b which showed much higher potency (K(i)=4.6 nM, compared with 2b, K(i)=230 nM) in which the 7-position of the imidazolo[1,2-a]pyrimidone core was substituted with a methyl group, and the ester at the 6-position was replaced by the 3-methoxyphenyl group.