Nuclear volumes and the frequencies of nuclei of different shape classes were determined, in cells in interphase and in prophase, in meristems of young and old primary roots, of long lateral roots and of small primordia ofVicia faba.Protein content of isolated nuclei of lateral roots was also determined. Lateral roots were treated with 0.0002% colchicine for two hours and volumes of metaphase chromosome complements were determined. The parameters measured all showed considerable variation: 1) interphase nuclear volumes covered an 8-fold range in old primaries and a 14-fold range in lateral roots; 2) nuclear protein contents showed a 5-fold spread of values; 3) prophase nuclear volumes showed a 4 to 7-fold range of values; 4) chromosome volumes covered a 5-fold range. Nuclear volumes, in most cases, were significantly different from a linear or a log normal distribution. Nevertheless, these values, on probit plots, did not fall into discrete sub-populations and we were unable to distinguish between fast, slow and non-cycling cells on the basis of their nuclear volumes. This and the variability in nuclear volume, suggests that nuclear growth rates are not uniform in interphase. It was concluded that since nuclear volumes are so variable they cannot be used to estimate the age of a particular cell in the cell cycle. Meristematic cells are heterogeneous for cell cycle duration and for a number of nuclear parameters.