Nuclear DNA, nuclear protein and nuclear size have been measured in cells of the cortex, pith and vascular tissue from three successive internodes in the stem of Pisum sativum. New techniques of computer-linked cytophotometry were used to measure these parameters simultaneously in both section and squash preparations. In cortical cells no endoreduplicated nuclei were seen in the internodes measured. In cortical cells from the oldest internode measured, a population of large nuclei with the 2C DNA amount was observed which was not present in the younger internodes. In the oldest pith nuclei measured a few 8C nuclei were present, but maturing pith was most characterized by increasing nuclear size and the population of nuclei accumulating with the 4C DNA amount. Polyploid nuclei were present in all of the vascular tissue measured, including the youngest internode. Maturing vascular tissue was also characterized by increasing nuclear size. Nuclear protein measurements demonstrated a close link between nuclear protein and nuclear size and suggest that increased nuclear size, with constant DNA content, may be due to increased nuclear protein. This raises the question of the nature and function of this nuclear protein, perhaps more characteristic of differentiating cells than dividing cells.