Syntactic reconstruction has long been virtually outlawed in historical-comparative research, more or less ever since Watkins’s influential works on the problems of reconstructing word order for Proto-Indo-European. Recently, through the emergence of Construction Grammar, where complex syntactic structures are regarded as form–function pairings, a resurgence of syntactic reconstruction is made possible, as complex syntactic structures become a legitimate object of the Comparative Method. Given the legitimacy of syntactic reconstruction, and hence the possible reconstruction of argument-structure constructions, a major question arises as to whether grammatical relations are also reconstructable for earlier undocumented language periods. We argue that if the constructions singling out grammatical relations can be reconstructed for a proto-branch, the grammatical relations following from these are also reconstructable for that proto-branch. In order to illustrate our methodology, we show how a reconstruction of the subject function in Proto-Germanic may be carried out, more specifically of oblique-subject predicates like ‘hunger’, ‘thirst’ and ‘lust’, based on the subject properties found in the earliest Germanic daughter languages.