A series of 8-substituted derivatives of adenosine monophosphate (AMP) and inosine monophosphate (IMP) were synthesized and examined for their ability to inhibit Escherichia coli IMP dehydrogenase. All compounds studied were competitive inhibitors in IMP-dependent competition studies and lacked substrate activity. In oxidized nicotinamide adenine dinucleotide dependent studies, 8-(p-NO2PhCH2S)-IMP was noncompetitive and 8-(p-NO2PhCH2S)-AMP showed mixed inhibition. Multiple regression analysis showed that for the series of 8-(para-substituted-benzylthio)-AMPs and -IMPs, the electron-withdrawing ability of the para substituent on the benzylthio moiety correlated best with log Ki of the analogues.