SUMMARYTwo distinct phenomena occur when water is added to air-dry lichens: (1) an immediate nonmetabolic release of a substantial volume of gas containing 75-80% COj; and (2) a rapid rise in respiration to rates well above control values. This excess respiration ('resaturation respiration') persisted for ahout 9 hours in Peltigera polydactyla and about 2 in Xanthoria aureola and was, unlike basal respiration, azidc-and cyanide-sensitive. When the mannitol content of Peltigera discs had been doubled by feeding glucose before drying, the initial rate of subsequent resaturation respiration was correspondingly much higher. Cycles of wetting and drying caused progressive reductions both in mannitol content and resaturation respiration. In Peltigera, resaturation respiration occurred if the water content of discs had fallen below approximately 40% of maximum. Xanthoria aureola, which grows in drier habitats than Peltigera polydactyla, was much more tolerant of cycles of wetting and drying and showed much less intense resaturation respiration.