The hemodynamic and biochemical properties of bisoprolol, a new highly beta 1-adrenoceptor selective blocker, were compared with those of atenolol. The studies were performed in eight healthy volunteers and consisted of two separate parts. In the first part the effects of oral doses of 20 mg bisoprolol, 100 mg atenolol, and placebo, given in a double-blind randomized order, on the hemodynamic and biochemical changes induced by multiple doses of intravenous isoproterenol and bicycle exercise were investigated. Blood pressure and heart rate and the plasma concentrations of magnesium, potassium (K+), glucose, lactate, renin activity, norepinephrine, and epinephrine were determined. In the second part the effects of the same oral doses of bisoprolol and atenolol on changes in forearm blood flow (FBF) induced by intra-arterial (i.a.) infusions of isoproterenol and epinephrine were investigated. After both beta blockers approximately 10-times higher doses of isoproterenol i.v. were needed to obtain similar hemodynamic responses as after placebo. The hemodynamic and biochemical changes during isoproterenol i.v. and bicycle exercise were almost identically influenced by bisoprolol and atenolol. However, after bisoprolol, isoproterenol lowered plasma K+ more (p less than 0.02) and the exercise-induced increase in renin activity was less (p less than 0.02) than after atenolol. The effect of isoproterenol i.a. on FBF was attenuated to the same extent by bisoprolol and atenolol. The vasodilatory component of epinephrine i.a. was reduced only by atenolol (p less than 0.05). It is concluded that bisoprolol and atenolol are selective beta 1 antagonists. In the doses used, bisoprolol showed only slightly more potency and selectivity for beta 1 adrenoceptors than atenolol.