Abstract: This invention relates to a process for preparing derivatives of 5,11-dihydro-6H-pyrido[2,3-b][1,4]-benzodiazepin-6-one of general formula (I) ##STR1## in which R is hydrogen, halogen or methyl;R.sub.1 and R.sub.2 are linear or branched C.sub.2 -C.sub.4 alkyl groups, which alternatively may be joined together to give a piperazine cycle which can be substituted in position 4 with a methyl, ethyl or benzyl group,and their salts with organic and inorganic acids, characterised by comprising the following stages:(a) reacting a compound of general formula (X), ##STR2## in which R has the meaning heretofore defined, with 2-cloro-3-amino-pyridine of formula (III) ##STR3## in the presence of polyphosphoric acid, to give an intermediate of general formula (II) ##STR4## (b) reacting said intermediate with a compound of general formula (XI) ##STR5## in which R.sub.1 and R.sub.2 have the meaning heretofore defined and R.sub.