When, during its industrial propagation, baker's yeast is transferred from high‐nutrient anaerobic growth conditions to vigorously aerated, low‐nutrient conditions its nicotinic acid and NAD‐NADH2* contents decrease to two‐fifths of the original amounts. The content of nicotinic acid falls from about 8.2 μmoles to 3.2 μmoles and the NAD‐NADH2 content from 6.6 μmoles to 2.7 μmoles per g. dry matter. The amount of nicotinic acid synthesized remains almost at the same level when calculated per g. yeast dry matter, being increased threefold when calculated as a percentage of the total amount of nicotinic acid present in the yeast. Hence, the fall in the nicotinic acid content is not so steep as would be expected from the limited supply of molasses, containing nicotinic acid. Baker's yeast is able to take up nicotinic acid without difficulty from the growth medium, accumulating nicotinic acid up to 8, or even 14 times the content of the original yeast. Addition of nicotinic acid inhibits the yeast's own nicotinic acid synthesis, and may stop it completely. When an amount of nicotinic acid three times as great as the content in seed yeast was added to an aerated culture at the commercial yeast stage, the nicotinic acid and NAD contents were doubled in 1 hr. Later, the content of NAD fell abruptly, and at the end of the growth period was at the same level as in a parallel propagation to which nicotinic acid had not been added. This seems to indicate that, under aerobic conditions, baker's yeast does not “need” more NAD than it contains after the normal propagation process. Laboratory experiments proved that although the content of nicotinic acid in baker's yeast increased to as much as 14 times its original amount, the content of NAD only rose two‐fold.