Feeding gametophytes of Pteridium aquilinum with tritiated uridine, followed by autoradiography, revealed two waves of incorporation of the nucleoside into nuclei during oognesis. The first, affecting the nucleus of the primary cell of the archegonium, is interpreted as indicating the activation of the genes initiating oogenesis. The second, seen in the nucleus of the maturing egg, is believed to be concerned with the differentiation of the egg itself.The results of the autoradiography, corroborated by enzyme digestion and fluorescence microscopy, also demonstrated a high concentration of ribonucleic acid towards the periphery of the mature egg, some possibly located in nucleolus-like bodies found only in this region of the cytoplasm. It is suggested that part of this cytoplasmic ribonucleic acid may represent genetic information remaining untranslated until after fertilization.No evidence was found of any asymmetry in the distribution of the ribonucleic acid in mature eggs which might account for the polarity of developing zygotes.