Fiction reading is a popular leisure activity associated with a variety of pleasurable experiences, including suspense, narrative transportation, and—as indicated by recent empirical studies—also flow. In the context of fiction reading, flow—generally defined as a pleasurable state of mind experienced during an optimally stimulating activity—is specifically related to an optimal balance between text‐driven challenges and the reader’s capabilities in constructing a mental story model. The experimental study reported here focused on the psychophysiological underpinnings of flow in the reading context. Cardiovascular data were collected from 84 participants both during a relaxation baseline prior to reading and during reading. Participants were randomly assigned to read one of three versions of a chapter from Homer’s Odyssey. According to statistical readability indices, these versions were low, intermediate, or high in readability, and hence in cognitive challenge. Flow was measured immediately after reading with a self‐report scale that was tailored to assess reading‐specific flow experiences. Regression analyses revealed that cardiovascular activation patterns measured before reading that are reflective of parasympathetic dominance—that is, an inner state associated with relaxation and cognitive fluency—moderated flow experiences during reading. In line with the stipulations of flow theory in regard to matching challenge levels being the key determinant for flow, this pattern supported subsequent flow experiences only in response to text versions of high or intermediate, but not of low cognitive challenge. Differences in cardiac vagal tone during reading were, however, not sensitive to our experimental modifications and not predictive of flow experiences.