The distribution of perichromatin granules (PGs), a storage form of pre-mRNAs, was studied in supraoptic neurons of control and osmotically stimulated rats, and also after treatment with cycloheximide, a protein-synthesis inhibitor. In non-cycloheximide-treated rats, neuronal activation by dehydration significantly decreased the number of PGs. Conversely, PGs were drastically increased in the supraoptic neurons of dehydrated rats treated with cycloheximide for 4 h. This suggests that cycloheximide does not interfere with the transcriptional activation induced by dehydration, but it affects the processing of newly synthesized pre-mRNAs. Moreover, protein-synthesis inhibition was associated with the formation of intranuclear bundles of tubular filaments.