In the article the material of monumental texts of the Ukrainian language of the 16th – the first half of the 17th century (business documents, artistic, polemical, chronicle, scientific and confessional literature) and the studied monuments of the Old Belarusian language are studied the diachronic aspect of the use of case forms (generic or accusative) in negative verb constructions; their functioning in modern Ukrainian and Belarusian languages is analyzed. It has been revealed that in the monuments of the Ukrainian language of the specified period the accusative case in denial is inferior to the generic one. The use of certain syntactic models (parallel use of genitive and accusative forms in the pre- and postposition) was determined by the general style and place of writing the monuments. In studies of monuments of the Old Belarusian language in this position the genitive was fixed, and sometimes in negative constructions the accusative and the genitive were allowed at the same time. In the linguistics of the 20th – early 21st century philologists have repeatedly drawn attention to the peculiarities of the use of genitive and accusative cases in negative constructions both on the All-Slavic background and on the material of individual languages. Synchronously, it has been revealed that in the modern Ukrainian language the literary norm in negative constructions is the use of the genitive case instead of the accusative. However, there is no noticeable tendency to replace the accusative and the genitive in verbs with a negative participle not, as there are many cases of using the possessive case in literature and in everyday speech. On the other hand, there is no unanimity in the grammars of the modern Belarusian language on this issue: some scholars believe that both generic and possessive cases are possible in negative constructions, while others believe that only generic is possible. However, from a sample of analyzed works of Belarusian writers of the twentieth century, artistic and journalistic posts, as well as conversational style records, it can be argued that there are only a few cases of use of the accusative case, in particular in proverbs and sayings, and only the genitive is dominant in the negative constructions.