Frequent words tend to shorten (see e.g. Schuchardt 1885, Hooper 1976), as do words that have a high probability of occurrence given a neighboring word (Jurafsky et al. 2001). This tendency has been cited in support of the claim that probabilities are an inherent part of grammar, and of syntax in particular. There is widespread consensus, however, that the syntax of natural languages cannot be captured in terms of item-to-item transitions (Chomsky 1957). Therefore, unless one considers probabilities of syntactic structures, rather than particular combinations of neighboring words, pronunciation variation cannot be said to reflect probabilistic effects in syntax. In this article, we report a case of pronunciation variation that reflects contextual probabilities of syntactic structures. The relevant probabilities are based on the probability of a given syntactic structure, given a particular verb. We showthat these probabilities affect American English /t,d/- deletion, as well as the durations of words and phrases. Our results are consistent with the notion that knowledge of grammar includes knowledge of probabilities of syntactic structures, and that this knowledge affects language production.