Angiotensin converting enzyme (ACE) inhibitors and angiotensin II (AII) type 1 receptor antagonists have strong cytostatic properties on in vitro cultures of many normal and neoplastic cells. They are effective, in particular, in reducing the growth of human lung fibroblasts, renal canine epithelial cells, bovine adrenal endothelial cells, simian T lymphocytes, and of neoplastic cell lines derived from human neuroblastomas, a ductal pancreatic carcinoma of the Syrian hamsters, human salivary glands adenocarcinomas, and two lines of human breast adenocarcinomas. ACE inhibitors are also effective in protecting lungs, kidneys and bladders from the development of nephropathy, pneumopathy, cystitis, and eventually fibrosis in different models of organ-induced damage such as exposure to radiation, chronic hypoxia, administration of the alkaloid monocrotaline or bladder ligation. ACE inhibitors and AII type 1 receptor antagonists are also effective in reducing excessive vascular neoformation in a model of injury to the cornea of rats and rabbits, and in controlling the excessive angiogenesis observed in the Solt-Farber model of experimentally induced hepatoma, in methylcholantrene or radiation-induced fibrosarcomas, in radiation-induced squamous cell carcinomas and in the MA-16 viralinduced mammary carcinoma of the mouse. Captopril was, in addition, effective in controlling tumor growth in a case of Kaposis sarcoma in humans. The inhibition of AII synthesis and/or its blockade by AII receptors is likely to be an important mechanism for this cytostatic action. The mitogenic effect of AII is well established and a reduction of AII synthesis may well explain cell and neoplasm delayed growth. Moreover, AII regulates and enhances the activity of several growth factors including transforming growth factor B (TGFB) and smooth muscle actin (SMA), and many of these factors are reduced in tissues of animals treated with ACE inhibitors and AII type 1 receptor antagonists. These processes seem to be particularly relevant in the control of fibroblast growth and in the control of the ensuing fibrosis. The ACE inhibitors containing a sulphydril (SH) or thiol radical in their moiety (Captopril and CL242817) seemed to be more effective in controlling fibrosis and the growth of some neoplastic cells than those ACE inhibitors without this thiol radical in their structure, even if the second group of these drugs show in vitro a stronger inhibitory effect on converting enzyme activity. Pharmacologically it is known that ACE inhibitors containing a thiol radical also have antioxidant properties and they are efficient in controlling metalloproteinase action. However, although these additional properties are pharmacologically relevant, the blockade of AII synthesis plays an essential role in the cytostatic activity of these two categories of drugs. These observations underline that in addition to the beneficial effect of these drugs on the cardiovascular system, new potential applications are opening for their wider deployment.