The formation of protochlorophyllide and protochlorophyllide phytyl ester was investigated during etioplast biogenesis in order to study the biosynthetic relation of these two compounds. Protochlorophyllide accumulates slowly during the first 2 days of germination, its rate of formation increases sharply during the 3rd day, and then it decreases. Protochlorophyllide phytyl ester starts accumulating a day later; its formation coincides with the initiation of xanthophyll biosynthesis. Kinetic analysis of specific radioactivities after (14)C labeling of the protochlorophyll pools does not support the currently accepted conversion of protochlorophyllide into protochlorophyllide phytyl ester, but suggests that both compounds originate simultaneously from a common precursor pool.