Gibberellin- and auxin-induced elongation in the cucumber hypocotyl was associated with a substantial increase in DNA content. However, the increase in number of cell nuclei was relatively much smaller than would be expected on that basis. Radioautographic examinations showed that 3H-thymidine was incorporated into nearly all chloroplasts, while very few cell nuclei were labelled in the same preparations. Determinations of total DNA content plus measurements of 3H-TdR incorporation in the nuclear and non-nuclear fractions indicated that the specific rate of DNA synthesis was 10–20 times greater in the non-nuclear fraction than in the nuclear one. It was concluded, therefore, that most of the hormone-enhanced DNA synthesis in this primarily non-dividing tissue takes place outside the nucleus, in DNA containing organelles, such as chloroplasts and mitochondria.