A series of dinucleotides, analogous to nicotinamide adenine dinucleotide but containing the five-membered base nucleosides tiazofurin (1a), selenazofurin (1b), ribavirin (2), and AICAR (3) in place of nicotinamide ribonucleoside, were prepared in greater than 50% yield by reacting the corresponding nucleotide imidazolidates (6a-d) with adenosine 5'-monophosphate (AMP). The symmetric dinucleotides of tiazofurin (TTD, 8e) and selenazofurin (SSD, 8f) were also prepared by a similar methodology. These dinucleotides were characterized by 1H NMR and fast atom bombardment MS and were evaluated for their inhibitory potency against a partially purified preparation of tumoral inosine monophosphate dehydrogenase (IMPD) from P388 cells. The order of potency found was SAD (8b) greater than TAD (8a) much greater than SSD (8f) congruent to TTD (8e) congruent to RAD (8c) much much greater than ZAD (8d). On kinetic analysis none of the dinucleotides produced competitive inhibition of IMPD with NAD as a variable substrate. In addition to their superior IMPD inhibitory activity, SAD and TAD appear to be the only dinucleotides, besides NAD, that are formed naturally by the NAD pyrophosphorylase from P388 lymphoblasts.