Enzymatically separated tobacco leaf cells took up amino acids, uracil, and uridine from the incubation medium and incorporated them into proteins and RNA, respectively, at a linear rate for approximately 30 hours. Both uptake and incorporation were light-dependent, although cells prepared from preilluminated plants or preillumination of cells allowed some uptake and incorporation to occur in the dark. The light was necessary to satisfy a photosynthetic requirement, but could be replaced in part by ATP in the medium.Several lines of evidence support the conclusion that the rate of uptake of amino acids, uracil, and uridine was dependent upon the subsequent incorporation of these compounds into macromolecules.