 The Fourth Doctor is briefly aged 500 years in "The Leisure Hive", leaving him an old man but still active.<ref></ref> A similar situation occurred in "The Sound of Drums", where the Master uses specially made technology to age the Tenth Doctor by a century, leaving him in a frail and helpless state.A further application of this technology in the following episode, "Last of the Time Lords", suspends the Doctor's capacity to regenerate, showing the effects of 900 years of life without regeneration.<ref name="Last of the Time Lords"></ref>

In "The Two Doctors",<ref name="Two Doctors"></ref> the Second Doctor states that the "Rassilon Imprimatur" allows Time Lords to safely travel through time, becoming symbionts with their TARDISes, and that the reason other species are incapable of developing time travel is that they lack the imprimatur.However, he implies later that he was lying about at least some of this information to mislead the Sontarans.