1. Pindolol is a beta-adrenoceptor blocking drug with ISA (also called partial agonist activity). This means that in addition to blocking the effects of beta-adrenoceptor agonists, it produces some stimulation of beta-adrenoceptors. 2. In vitro studies with pindolol show that its maximum stimulant action is similar to that of isoprenaline in tissues possessing mainly beta 2-adrenoceptors, but is negligible in tissues possessing mainly beta 1-adrenoceptors. This suggests selective stimulation of beta 2-adrenoceptors. 3. In man the arteriodilator effects observed after intra-arterially infused pindolol at concentrations within the same range as those producing an antihypertensive effect also suggest a stimulant action on vascular beta 2-adrenoceptors. 4. The fact that pindolol prevents the reduction of resting heart rate and cardiac output observed after drugs lacking ISA at first sight suggests stimulation of cardiac beta 1-adrenoceptors. However, human atria possess not only beta 1- but also beta 2-adrenoceptors, stimulation of which would produce the same effect. 5. Although all beta-adrenoceptor antagonists lower blood pressure, recent experiments have shown that those agents with combined beta 1-adrenoceptor blocking activity and ISA at those receptors are less effective. This observation lends weight to the thesis that pindolol does not stimulate beta 1-adrenoceptors since it lowers blood pressure as effectively as drugs lacking ISA. 6. The evidence available therefore suggests that although pindolol blocks both beta 1- and beta 2-subtypes, it selectively stimulates beta 2-adrenoceptors.