Summary Field studies of a tropical tree, Anacardium excelsum , and a northern hemisphere high altitude fern, Cryptogramma crispa , revealed marked diurnal changes in soluble flavonoid content of leaves and fronds, respectively. The flavonoid content increased during the morning and decreased during the afternoon. In plants of C. crispa covered with UV-B absorbing filters, the flavonoid content remained at a constant level throughout the day/night cycle. Upon removal of UV-B absorbing filters (at night), the flavonoid content increased the next morning in a fashion similar to that observed in control plants maintained without filters. Decreases in photosystem II photochemical efficiency upon exposure of C. crispa to natutal daylight were similar in plants previously covered with UV-B absorbing filters and in control plants, probably owing to the observed ability of plants to rapidly accumulate UV-B protective flavonoids.