The time of regaining photosynthetic activity was shown to be extremely fast after water was added to the thallus of desiccated Lobaria pulmonaria. Rapid increase of the fluorescence yield evidenced a recovery of photosynthetic activity within less than 20 seconds. X-ray microanalysis of thalli re- hydrated with a solution containing tracers for apoplastic (La) and cell-to cell- transport (Rb) showed that the water was translocated in the apoplast, irrespective whether the fungal hyphae were thick-walled as in L. pulmonaria or thin- walled as in Xephroma bellum. The data give no evidence for a substantial cell- to-cell transport during rapid rehydration. Cavitated cells near the thallus surface were refilled within one second, but other cortical and medullary hyphae were only subsequently rehydrated within 10 and 70 s, respectively. The photo- biont cells regained their globular shape only after 80 seconds. Pulse-amplitude modulated chlorophyll fluorescence and low-temperature scanning microscopy enabled the first time a one-second time resolution of the functional and structural processes during the rehvdration of desiccated lichen thalli