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The activities of nuclear enzymes involved in NAD+ metabolism in Saccharomyces cerevisiae strain 913a-1 and its mutant 110 previously selected as an NAD+ producer were investigated. The presence of extracellular nicotinamide increased the total NAD+ pool in the cells and increased [3H]nicotinic acid incorporation; however, NAD+ concentration in isolated nuclei decreased slightly. The stimulating effect of nicotinamide on intracellular synthesis of NAD+ correlated with increases in ADP-ribosyl transferase, NAD+-pyrophosphorylase, and NAD+ase activities.