Ultrastructural nucleolar reorganization induced by toxic and thermal stress in different experimental models are studied. Adriamycin-treated cardiac and skeletal cell in vivo as well as embryonic chick heart cells in vitro and thermal stressed human hepatoma cell line showed compact nucleoli, distinct signs of nucleolar alteration and segregation, including a ring-like arrangement of nucleolar components. Stress may cause dynamic and nonpermanent changes in the nucleolus which might represent a tissues-type and stress-type independent cellular mechanism that needs to be determinedConsejo de Desarrollo Científico y Humanístico de la Universidad Central de Venezuela; Laboratorios Elmor de Venezuela