Nicotinamide (NIC), at a concentration of 8.2 mM, increased the glutathione content to 300% of the control in an anthocyanin accumulating Catharanthus roseus (periwinkle) tissue culture when harvested 10 days after NIC addition. One day after NIC addition, the content of oxidized glutathione (GSSG) increased to 169% of the control, but the contents of total glutathione (GSHtot) and reduced glutathione (GSH) were unchanged. Longer NIC exposure resulted in increasing contents of GSHtot and GSH, as well as GSSG and a concomitant increase in anthocyanin accumulation. The glutathione biosynthesis inhibitor, buthioninesulfoximine (BSO), decreased glutathione levels in the tissue and offset the NIC-induced increase in glutathione content. BSO also decreased the NIC-induced anthocyanin accumulation in C. roseus tissue culture.