Thereafter, sodium nitroprusside is infused to reduce MAP by 20% (15%-25%) and 40% (35%-45% and minimally MAP 50 mmHg at a maximal infusion rate of 10 µg/(kg*min)). Sodium nitroprusside is a short-acting, potent vasodilator that activates guanylate cyclase directly or by production of nitric oxide, causing relaxation of smooth muscle cells in arteries and veins. The effect of sodium nitroprusside is attained within 2 min, and the half-life is 2 min whereby the effect is short lasting. Sodium nitroprusside has no direct effect on CBF. Infusion of sodium nitroprusside is by an electronic infusion pump, at 0.25 µg/(kg*min) for one minute and increased by 0.25 µg/(kg*min) each minute. The effect on MAP is evaluated each minute before the infusion speed is increased. When a level of MAP is reached, MAP is maintained for another minute where after measurements are conducted during 2-3 min. When evaluations have been done at both levels of MAP, the infusion speed is reduced gradually in order to avoid so-called "rebound hypertension". At this time the experiment is finished and catheters are removed.