Plasma catecholamine (CA) were measured in August and Wistar rats before and during immobilization. A catheter was inserted into the tail artery to permit direct measurement of blood pressure (BP). The BP was the same in August and Wistar Understress as in control rats. The CA level was higher in August rats. Adrenaline and noradrenaline contents differed in rats of these strains which suggested different release of CAs from the adrenal medulla and sympathetic neurons in these rats during immobilization. The stability of BP under emotional stress was mostly correlated with the lower control level of plasma CAs and with their slightly increased level during immobilization.