The renin-angiotensin system is associated with a variety of pathophysiological processes in many organ systems, and is known to be involved in the normal regulation of blood pressure and in the pathogenesis of renovascular hypertension. Angiotensin II is a multifunctional hormone that manifests its properties by interacting with two major subtypes of cell surface receptors (AT1 and AT2). Angiotensin converting enzyme (ACE) inhibitors are able to modify the actions of the renin-angiotensin system, and are indicated for the treatment of hypertension and heart disease. The antihypertensive effects of ACE inhibiting drugs are related to their ability to block the conversion ofthe decapeptide, angiotensin I, to the potent pressor octapeptide, angiotensin II. ACE inhibitors have been implicated in fetopathies in humans and perinatal mortality in rats, rabbits, sheep and baboons. Human fetopathies were seen when ACE inhibitors were given around the 26th week of gestation. The major adverse effects in babies include: oligohydramnios, renal tubular dysgenesis, neonatal anuria, calvarial and pulmonary hypoplasia, mild to severe intrauterine growth retardation, persistent patent ductus arteriosus and fetal or neonatal death. These developmental anomalies are thought to be partly due to a direct action of ACE inhibitors on the fetal renin-angiotensin system and partly due to the ischemia resulting from matemal hypotension and decreases in fetal-placental blood flow and oxygen/nutrient delivery to the fetus. The purpose ofthis review is to briefly discuss the pathophysiological role ofthe reninangiotensin system, the therapeutic uses of ACE inhibitors in pregnant patients and to focus primarily on the major fetotoxic effects of ACE inhibitors encountered in humans and animal models. I will also review our recent data which show that capozide (captopril + hydrochlorothiazide) not only produces oligohydramnios but also disturbs the balance of glucose and NaCl in the maternal plasma and amniotic fluid of the rat.