The distribution of cellular and nuclear volume in synchronous populations of NHIK 3025 cells, which derive from a cervix carcinoma, have been measured by electronic sizing during the first cell cycle after mitotic selection. Cells given an X-ray dose of 580 rad in G1, were also studied. During the entire cell cycle the volume distribution of both cells and nuclei is an approximately Gaussian peak with a relative width at half maximum of about 30%. About half of this width is due to imperfect synchrony whereas the rest is associated with various time invariant factors. During S the mean volume of the cells grows exponentially whereas the nuclear volume increases faster than for exponential kinetics. Hence, although cellular and nuclear volumes are closely correlated, their ratio does not remain constant during the cell cycle. Volume growth during the first half of G1 is negligible especially for nuclei where the growth appears to be closely associated with DNA-synthesis. For unirradiated cells the growth of cellular and nuclear volume is negligible also during G2 + M. In contrast, the X-irradiated cells continue to grow during the 6 hr mitotic delay with a rate that is constant and about half of that observed in late S. Hence, radiation induced mitotic delay does not appear merely as a lengthening of an otherwise normal G2. During G1 and S the irradiated cells were identical to unirradiated ones with respect to all the parameters measured.