Why is semantic change in grammaticalization typically unidirectional? It is a well-established finding that in grammaticalizing constructions, more concrete meanings tend to evolve into more schematic meanings. Jäger & Rosenbach (2008) appeal to the psychological phenomenon of asymmetric priming in order to explain this tendency. This article aims to evaluate their proposal on the basis of experimental psycholinguistic evidence. Asymmetric priming is a pattern of cognitive association in which one idea strongly evokes another (i.e. paddle strongly evokes water), while that second idea does not evoke the first one with the same force (water only weakly evokes paddle). Asymmetric priming would elegantly explain why semantic change in grammaticalization tends to be unidirectional, as in the case of English be going to, which has evolved out of the lexical verb go. As yet, empirical engagement with Jäger & Rosenbach's hypothesis has been limited. We present experimental evidence from a maze task (Forster et al.2009), in which we test whether asymmetric priming obtains between lexical forms (such as go) and their grammaticalized counterparts (be going to). On the asymmetric priming hypothesis, the former should prime the latter, but not vice versa. Contrary to the hypothesis, we observe a negative priming effect: speakers who have recently been exposed to a lexical element are significantly slower to process its grammaticalized variant. We interpret this observation as a horror aequi phenomenon (Rohdenburg & Mondorf 2003).