Castor bean seeds, removed from the developing fruit at the very beginning of the seed dehydration phase, easily germinate when decoated and put on water soaked filter paper at 27° C. The present experiments deal with the changes in RNA and in the activity of citochrome oxidase, malate dehydrogenase and succinate-citochrome c-reductase in the mitochondrial fraction from ripening seeds, removed at this stage, maintained 48 hours under various hydration conditions, and finally allowed to germinate under conditions of unlimited water availability. During the first 24 hours from the removal of the seed from the fruit, a significant drop of the enzyme activities as well as of the nitrogen and of the RNA contents in the mitochondrial fraction was observed, under any condition of hydration (unlimited water availability, hydric equilibrium in a moist environment, or mild or strong dehydration). In the following 24 h period enzyme activities and RNA start to increase, in the presence of unlimited water, while they go on decreasing under the conditions of hydric equilibrium and of dehydration. When, after 48 h of the above treatments, all seeds were put in the presence of unlimited water, the rise of enzyme activities and of RNA continued in the seeds previously mantained in unlimited water, and progressively appeared in the seeds pretreated under the two other conditions. The $Q_{O_{2}}$ of the intact seeds closely paralleled the behaviour of the mitochondrial enzyme activities. These findings suggest that a water insensitive phase of degradation of the mitochondrial system might be required to allow the transition of the mitochondrial system of the maturing seed to a situation in which water availability is the limiting factor for the further evolution, characteristic of germination.