Murine lymphoid cell lines and rat thymocytes treated in vitro with glucocorticoid hormones provide a convenient system for studying the nuclear changes in apoptosis. Morphologically the nucleolus disintegrates and chromatin undergoes an unusual generalized condensation. This is associated with excision of most of the nuclear DNA to short but well-organized chains of nucleosomes, apparently by an endogenous non-lysosomal nuclease. The process is dependent upon macromolecular synthesis and probably is mediated, at least remotely, by the classical steroid-receptor-gene activation pathway. A similar process of chromatin condensation and excision can be produced by the calcium-magnesium ionophore A23187. In other circumstances of 'programmed cell death', analogous chromatin condensation, excision and requirements for macromolecular synthesis have been documented.