Pyridine nucleotides are key redox carriers in the soluble phase of all living cells, and both NAD and NADP play crucial roles in pro-oxidant and antioxidant metabolism. Recent data also suggest a number of non-redox mechanisms by which these nucleotides could influence cell function. In cases where these mechanisms involve NAD(P) consumption, resynthesis must occur to maintain nucleotide pools. Important information on the pathways involved in NAD synthesis in plants is beginning to appear, but many outstanding questions remain. This work provides an overview of the current state of knowledge on NAD synthesis pathways in plants and other organisms, analyses plant sequences for the first two enzymes of the de novo synthesis of NAD, proposes a preliminary model for the intracellular distribution of NAD synthesis, presents plant homologues of recently identified yeast mitochondrial NAD transporters, and discusses factors likely to be important in the regulation of NAD synthesis and contents in plants, with particular reference to stress conditions.