There are currently no consensus criteria to define the dosage of intra-arterial Nimodipine to be used to obtain permanent remission of vasospasm secondary to subarachnoid haemorrhage. The purpose of this paper is to establish a mathematical relation between the number of vessels affected by this kind of vasospasm and the Nimodipine dose that must be used for intra- arterial angioplasty to obtain permanent remission of the vasospasm.