Compounds of the formula ##STR1## wherein A, B, C and D are each independently --N.dbd. or --CH.dbd., provided, however, that at least two of them must be --N.dbd., and when A and C are both --N.dbd., at least one or both of B and D must be --N.dbd.; andR.sub.1, R.sub.2 and R.sub.3 are substituents of diverse types;tautomers thereof; and non-toxic, pharmacologically acceptable acid addition salts thereof. The compounds are useful as cardiotonics.