With reference to the indirect evidence showing that heart growth in the presence of a cardiac overload is modulated by the adrenal cortex, we studied heart reactions to overloading (ligation of the abdominal aorta or hyperthyroidism) with simultaneous administration of aminoglutethimide, an inhibitor of steroidogenesis. Aminoglutethimide (Elipten CIBA, 0.5% in the diet) markedly inhibited the growth reaction of the heart 14 days after ligation of the abdominal aorta (reducing the aortal blood flow to about 40%) or after the administration of dried thyroid (Thyreoidin SPOFA, 0.2% in the diet). The effect of aminoglutethimide is evidently not due to its simultaneous antithyroid action, because methylthiouracil (Alkiron SPOFA, 0.2% in the diet) did not limit heart growth after ligation of the abdominal aorta, despite its having a stronger goitrogenic effect than aminoglutethimide. The results are a further contribution to the series of indirect evidence of the modulating effect of the adrenal cortex on heart growth.