The aim of this paper is to explore possible connections between ‘grammatical metaphor’ and ‘grammaticalization’, especially with a view to identifying synergies and fostering cross-fertilization of insights between SFL and ‘grammaticalization theory’. After a characterization of the concepts of grammatical metaphor and grammaticalization, it is argued that the two notions are intricately connected. Their connection is explored at two levels: a theoretical-conceptual and a descriptive level. At the theoretical-conceptual level, it is argued that metaphoricity and codification are two sides of the same coin, providing complementary perspectives on synchronic variation and diachronic ‘change’, which can be deconstructed if one changes the time depth or the angle of the perspective. At the descriptive level it is shown that interpersonal grammatical metaphors can be seen as a breeding ground for grammaticalization. Aspects of the syntagmatic nature of interpersonal metaphors are identified as bridging contexts for their grammaticalization into expressions of epistemic assessment, and two grammaticalization paths are identified on the basis of interacting vectors of grammaticalization and metaphoricity, viz. a path into discourse markers and one into epistemic adverbials.