In 36 healthy subjects of various ages (14-76 yr) the number of .alpha.2-adrenergic receptors in platelets, as determined by [3H]yohimbine binding, and plasma catecholamine levels were measured. A highly significant negative correlation (r = - 0.666, P < 0.001) between the number of .alpha.2-adrenergic receptors and age was found, but plasma catecholamine concentrations increased with increasing age. Reduced responses in the elderly to adrenergic stimuli may be due to reduced number of adrenergic receptors.