O(2) consumption by the desiccation-tolerant moss Tortula ruralis and the desiccation-intolerant Cratoneuron filicinum increased markedly during the latter stages of desiccation. ATP content of the mosses during desiccation was not correlated with O(2) consumption, but was influenced by the rate at which the mosses lost water. The more rapid the water loss, the more ATP that was present in the dry mosses. The pattern of O(2) consumption on rehydration also was influenced by the previous rate of desiccation. After rapid desiccation of T. ruralis O(2) consumption upon rehydration was considerably elevated, and for up to 24 hours. After very slow desiccation the elevation was small and brief. Normal O(2) consumption did not occur in C. filicinum after rapid desiccation, but did so within a few hours of rehydration after slower speeds of drying. ATP levels in T. ruralis returned to normal within 5 to 10 minutes of rehydration. In C. filicinum, increases in ATP were closely correlated with O(2) consumption. These observations are considered to be related to differential damage caused to mitochondria and to cellular integrity by different speeds of water loss. The desiccation-tolerant moss appears to be able to repair the severe damage imposed by rapid desiccation whereas the desiccation-intolerant moss cannot.