Important metabolic changes have been demonstrated in the endosperm of Castor bean seeds during maturation and germination. The early phase of germination is characterized by a rapid rise of the Oxygen uptake (I) and of the activity of a number of enzymes (2,3). The increase of enzyme activity is largely due to “ex novo” synthesis of enzyme proteins (4,5). Moreover a rapid synthesis of RNA and of the ribosomal apparatus has been shown (6). The behaviour of the endosperm in the ripening seed can be considered during two distinct phases: I) development and accumulation of storage materials; 2) progressive dehydratation leading to the state of complete ripeness. As shown in this laboratory, the second phase of ripening is characterized by the marked decrease of almost all enzyme activities, due in part to reversible enzyme inactivation (7), but also to irreversible inactivation and to extensive damage of cell structures (3, 8). These findings suggested that protein synthesis inhibition, possibly due to inactivation of some component of nucleic acid system, could play a primary role in seed maturation. The present report deals with the behaviour of nucleic acids in the endosperm during the process of ripening.