Digital personal assistants, like Amazon Echo and Google Home, have grown in popularity in recent years. We believe that digital personal assistants have the potential to persuade, as an assistant can leverage information about the user and utilise its connection to the users’ digital ecosystem. However, to our best knowledge, there is no research on the persuasive power of these voice-controlled assistants. We conducted a user study with 48 participants, in the context of physical activity, to examine the effect of the three persuasive principles: Suggestion, Virtual Reward, and Praise. Our findings reveal that digital personal assistants have persuasive power, and the three principles had different strengths. Virtual Reward was good at encouraging endurance, Suggestion was good at encouraging physical effort, and Praise created positive reassurance for beginners. We outline design considerations that should be taken into account when designing persuasive digital assistants and possible future research directions.