C O 2 fixation was studied in a lichen, Xanthoriaparietina, kept in continuous light, and with cyclic changes in light intensity, dark period or temperature. The diurnal and seasonal courses of CO 2 exchange were followed. The rate of net photosynthesis was observed to fall from morning to evening, and this decline was more pronounced in winter than in summer. The maximal net photosynthetic rate, 223ng CO2g-~dws -1, occured in winter and the minimum, 94ng CO2 g-ldw s -l, late in spring. The light compensation point in summer was four times as high as in winter. In continuous light (180 or 90/~mol photons m -2 s -I , 15 ยฐC) net photosynthesis decreased noticeably during one week, falling below the level maintained in a 12h light: 12h dark cycle. Photosynthetic activity did not decrease, however, in lichens held in continuous light (90/~mol photons m -2 s -1) with cyclic changes of temperature (12 h 20 ยฐC: 12 h 5 ยฐC). Active photosynthesis was also maintained in light of cyclically changing intensity (12 h: 12 h, 15 ยฐC) when night-time light was at least 75% lower than illumination by day. A dark period of 4 hours in a 24-h light:dark cycle was sufficient to keep CO 2 fixation at the control level. It seems that plants need an unproductive period during the day to survive and this can be induced by fluctuations in light and/or temperature.