Substituted 5H-pyrimido[5,4-b]indoles of Formula I ##STR1## wherein R.sup.2 is halogen; the oxadiazolyl group ##STR2## wherein R" is lower alkyl with up to 3 carbon atoms; C.sub.1-5 -alkyl, cycloalkyl, aralkyl, or aryl; OR.sup.I, SO.sub.n R.sup.I with n being 0, 1, or 2, or ##STR3## wherein R.sup.I is hydrogen, C.sub.1-5 -alkyl, cycloalkyl, aralkyl, or aryl; ##STR4## wherein R.sup.II and R.sup.III are hydrogen, C.sub.1-5 -alkyl, C.sub.3-5 -alkenyl, cycloalkyl, aralkyl, aryl, or, with the connecting N-atom, a 5- or 6-membered heterocyclic ring; andR.sup.A is hydrogen; the oxadiazolyl group ##STR5## wherein R" is lower alkyl with up to 3 carbon atoms; halogen, nitro, OR.sup.I, SO.sub.n R.sup.I with n being 0, 1, or 2, ##STR6## wherein R.sup.I is hydrogen, C.sub.1-5 -alkyl, cycloalkyl, aralkyl, or aryl; ##STR7## wherein R.sup.II and R.sup.III are hydrogen, C.sub.1-5 -alkyl, cycloalkyl, C.sub.3-5 -alkenyl, aralkyl, aryl, or, with the connecting N-atom, a 5- or 6-membered heterocyclic ring; orPO(OR).sub.2 wherein R is C.sub.1-5 -alkyl, exhibit strong affinity for binding to benzodiazepine receptors.The novel compounds are suitable for use in psychopharmaceutical preparations.