...Extracellular NAD+ and nicotinamide mononucleotide (NMN), and potentially their deamidated counterparts, nicotinic acid adenine dinucleotide (NAAD) and nicotinic acid mononucleotide (NAMN), may serve as precursors of intracellular NAD. However, it is still debated whether nucleotides enter cells directly or whether they are converted to nucleosides and bases prior to uptake into cells. ...NMN exhibited rather high stability in cell culture, but was partially metabolized to NR. Using pharmacological inhibitors of plasma membrane transporters, we also showed that extracellular cleavage of NAD+ and NMN to NR is a prerequisite for using these nucleotides to maintain intracellular NAD contents. We also present evidence that, besides spontaneous hydrolysis, NR is intensively metabolized in cell culture by intracellular conversion to Nam. Our results demonstrate that both the cultured cells and the culture medium mediate a rather active conversion of NAD+ intermediates. Consequently, in studies of precursor supplementation and uptake, the culture conditions need to be carefully defined.