The metabolism of RNA and protein in the gametophytes of bracken fern (Pteridium aquilinum) is affected by the quality of light in which they are grown. When sporelings were grown as two-dimensional gametophytes in blue light, particulate fractions separated from the sporelings exhibited greater incorporation of uridine-(3)H and leucine-(3)H into RNA and protein, respectively, than those from sporelings grown as one-dimensional protonema in red light. After various periods of exposure of gametophytes to red or blue light in the presence of uridine-(3)H, the nuclei-rich fraction showed the highest specific activity in RNA, and irrespective of incubation time, blue light was more effective than red light. The possibility that enhanced synthesis of RNA in the nucleus in response to blue light is significantly related to the morphological growth pattern of the gametophytes, is discussed.