The effect of trilostane, a competitive inhibitor of 3β-hydroxysteroid dehydrogenase, on adrenal tissue and blood pressure was studied in normotensive and spontaneously hypertensive rats. The drug was administered orally in doses of 10, 200 and 650 mg/kg/day for 2 and 4 weeks. Trilostane induced an increase in adrenal weight by lipoid transformation of the zona fasciculata. Simultaneously a reduction of retroperitoneal fat tissue was seen in all animals. A significant increase in blood pressure was observed under a trilostane medication of 650 mg/kg/day in normotensive as well as in spontaneously hypertensive rats. Blood pressure under lower doses did not increase significantly. Both increase of blood pressure and reduction of retroperitoneal fat tissue might be caused by increased release and synthesis of catecholamines.