This study presents evidence for a new enzyme, d-ribose-5-P reductase, which catalyzes the reaction: d-ribose-5-P + NADPH + H(+) --> d-ribitol-5-P + NADP(+). The enzyme was isolated from Adonis vernalis L. leaves in 38% yield and was purified 71-fold. The reductase was NADPH specific and had a pH