The effects of purine and pyrimidine nucleotides and of the nucleosides on partially purified IMP dehydrogenase [EC 1.2.1.14], XMP aminase [EC 6.3.5.2] and succino-AMP lyase [EC 4.3.2.2] activities of Bacillus subtilis M strain of derepressed cells were studied. IMP dehydrogenase was inhibited strongly by GMP, the end product, and by XMP, the direct product of the reaction under dilute concentrations of IMP and KC1. When the concentrations of IMP and KC1 were both high, ATP also was a strong inhibitor. The fact that Lineweaver-Burk plot against IMP curved upward in the presence of GMP and XMP and was linear in the absence indicated the enzyme was allosteric. XMP aminase utilized L-glutamine more effectively than ammonium sulfate as the amino donor and its activity was inhibited slightly by several ribonucleotides including GDP, GMP, adenosine, UDP, IDP, AMP and ITP. Succino-AMP lyase activity with succino-AMP as the substrate was inhibited competitively by the product, AMP, and its derivative, ATP.