This study investigated the relation between word surprisal and pupil dilation during reading. Participants' eye movements and pupil size were recorded while they read single sentences. Surprisal values for each word in the sentence stimuli were estimated by both a recurrent neural network and a phrase-structure grammar. Higher surprisal corresponded to longer word-reading time, and this effect was stronger when surprisal values were estimated by the neural network. In addition, there was an early, positive effect of surprisal on pupil size, from about 250 ms before word fixation until 100 ms after fixation. This early effect, which was only significant for the network-based surprisal estimates, is suggestive of a preparation-based account of surprisal.