To gain further insight into the supramolecular structure and behavior of chromatin, a set of parameters has been developed for the description of cell nuclear images. These images are obtained from Feulgen-stained nuclei in semithin sections. Data are extracted by a scanning microphotometer, printed on paper tape and processed by an electronic computer. The essential principles for the selection of descriptors were that: (a) they should correspond to characteristics that are employed in classical morphology and (b) they should be subject to interpretation in categories of biochemistry or molecular biology insofar as possible. Some of these parameters deal with the size and shape of the nucleus and its deoxyribonucleic acid content or staining behavior. Efforts were made to determine the relative amount of chromatin in the condensed state, because it may be assumed that the degree of condensation correlates with the activity of the chromatin. To demonstrate the power of the method, results from methylthiouracil-activated thyroid nuclei are presented. In stimulated nuclei the diffuse chromatin expands and fills the increasing nuclear volume, while the condensed chromatin retains its degree of condensation, and, probably, does not change in amount.