In the green frog, Rana esculenta, the mucin synthesis in the oviducal gland cells during the recovery phase (July-November) is joined with some morphological transformations of nuclei which progressively acquire a picnotic appearance. This phenomenon does not seem to be the expression of mucin synthesis control, since both at the beginning and at the end of the recovery phase the glandular cells show the same activity in vivo, as can be evaluated following the incorporation of labelled uridine into RNA and of labelled amino acids into proteins. The meaning of nuclei transformations and their relationship to the functions of oviduct glandular cells are not at present known.