Blood induced acute and chronic cerebral vasospasm were studied in Rhesus monkeys by serial angiography. Two vasoactive agents were evaluated. In acute spasm, phosphodiesterase inhibition by Aminophylline consistently reversed the vasospasm. This agent alone was ineffective in chronic spasm and required the addition of Isoproterenol to produce reversal of vasospasm. It is proposed that cerebral vasospasm may be associated with a decrease of intracellular c-AMP in vascular smooth muscle which can be reversed by manipulation of the enzyme pathway involved. It is further proposed that c-AMP is a basic common pathway through which adrenergic vascular reactivities are mediated.