Abstract The thyroid gland plays an important secondary role in the development of renal hypertension in the rat. Thyroid activity, as assessed by the ratio of thyroid weight to body weight, precipitate to serum I 131 ratio and mean thyroid acinar cell height, increases sharply as systolic blood pressure level approaches 150 to 160 mm. Hg i.e., the range of pressure usually considered to represent the high range of normotension and the low range of hypertension. The ratio of heart weight to body weight increases in a sigmoid-like fashion with increasing blood pressure and in an exponential fashion with increasing thyroid weight ratio. It is suggested that a mechanism (s) independent of thyroid gland activity increases blood pressure to levels of 150 to 160 mm. Hg. At this pressure level, thyroid gland activity increases and further elevates blood pressure to hypertensive levels.