Qui pro quo is an excellent means of creating a comic effect in ancient drama, particularly in the comedies of Plautus. In this study, qui pro quo is metaphorically transferred from drama to grammar and applied to the phenomenon of substituting one grammatical category for another. First, I will introduce the notion of canonical and non-canonical grammatical meaning, which is crucial for the idea of grammatical qui pro quo. The concept of oppositions and neutralisation will be proposed as a scientific hypothesis to explain why language employs this strategy. Then I will analyse the qui pro quo technique within the categories of tense and mood (e.g., Praesens pro futuro, Futurum pro praesente, Imperfectum pro praesente, Indicativus pro coniunctivo, Infinitivus pro indicativo, Indicativus pro imperativo, etc., nine patterns in all). After considering verbal categories, the sentential qui pro quo will be observed in five types of sentences with different illocutionary force. Finally, the effect of the phenomenon in question on the insubordinate ut-, si-, and quasi-clauses will be shown. In the end, I will summarise the findings and explain the mechanism of grammatical qui pro quo as manifestation of the neutralisation principle.