This article presents an experimental study that compares six expert and eleven novice interpreters in order to assess the effect of moderate and high delivery rates on the simultaneous interpreting of specialised discourse. The dependent variables were ear voice span (EVS) and target speech accuracy. The participants interpreted one medical speech from English into Spanish divided into three parts: the first and third parts were delivered at a moderate delivery rate and the second at a high delivery rate. Twelve segments were selected from the original speech and evaluated by five independent raters (two interpreters and three medical doctors) in terms of accuracy. The findings indicate that a high delivery rate has no significant impact on EVS, and no statistically significant differences were found between the EVS of the two groups of interpreters. The results also show that a high delivery rate has an impact on target speech accuracy in that both novices and experts perform better at a moderate delivery rate.