The present invention relates to thiadiazole derivatives having the formula: ##STR1## in which: R.sub.1 represents a phenyl group, non-substituted or substitued 1 to 3 times by a halogen atom, preferably chlorine or fluorine, or by a C.sub.1 -C.sub.4 alkyl group, preferably the methyl group, or by a C.sub.1 -C.sub.4 alkoxy group, preferably the methoxy group, or by a hydroxy group or by a trifluoromethyl group or a phenyl group substituted simultaneously by 1 to 3 halogen atoms and by 1 or 2 methyl groupings;R.sub.2 represents hydrogen or a C.sub.1 -C.sub.4 alkyl group,R.sub.3 represents an alkylamino group or a heterocyclic group,Application: Drugs for treatment of notably senile dementia.