1. The noradrenaline-induced accumulation of cyclic AMP is decreased in both the anterior and the posterior hypothalamus of hypertension-prone as compared with resistant rats, but is similar in the cortex.
2. The nonadrenaline-induced accumulation of cyclic AMP is decreased in the posterior hypothalamus as compared with the anterior hypothalamus of both strains.
3. The decreased sensitivity to phosphodiesterase inhibitor in the hypothalamus of hypertension-prone as compared with resistant rats suggests that strain differences in phosphodiesterase activity may exist.
4. Since differences in cyclic AMP were found in the hypothalamus and the medulla oblongata, regions involved in blood pressure control, but not in the cortex, they may be relevant to the diverse susceptibility to hypertension of hypertension-prone and resistant rats.