In the course of studies on the metabolic role of diguanosine tetraphosphate during development of Artemia salina, a guanosine monophosphate (GMP) reductase has been found in partially purified from the 150 000g Artemia cysts supernatant. From Lineweaver-Burk plots, two apparent Km values of 5 and 50 muM were obtained for GMP. Xanthosine monophosphate (XMP) is a very strong inhibitor of the reaction. In the presence of 1.5 muM XMP hyperbolic kinetics are found. Diguanosine tetraphosphate counteracts very effectively the inhibition of the activity by XMP, concomitantly changing to hyperbolic the kinetics of the enzyme, with a unique Km value of about 5 muM. The complex kinetic and the existence of allosteric e-fectors at physiological concentrations, together with our lack of success in resolving two isoenzymes, makes it very likely that GMP reductase presents negative cooperativity towards its substrate. The effect of diguanosine tetraphosphate on the enzyme is very specific; other structural analogues, diadenosine tetraphosphate and diguanosine triphosphate, tested a micromolar concentrations had no detectable effect on the enzyme. Guanosine triphosphate (GTP) (mM) was also able to counteract the inhibition of guanosine monophosphate (GMP) reductase by XMP. The properties of the Artemia GMP reductase are here compared with those of the similar enzyme from calf thymus and Escherchia coli. As a consequence, the regulation of eukaryotic GMP reductase is resulting to be quite different from that of the reductase from prokaryotes.