Oxadiazolopyrimidine derivatives of the formula ##STR1## wherein R and R.sub.1 are as hereinafter described, prepared, inter alia, by cyclizing a compound of the formula ##STR2## wherein R and R.sub.1 are as hereinafter described. The end products are useful in the treatment of vascular-conditioned hypertension or as vasodilators in the case of peripheral blood supply disorders.