It is first stated in The Deadly Assassin (1976) that a Time Lord can regenerate twelve times before dying (thirteen incarnations in all). There are exceptions to this rule, however; when a renegade Time Lord known as the Master finds himself at the end of his regenerative cycle, he takes possession of the body of another person to continue living (The Keeper of Traken, 1981), although he was using the Source of Traken to bind his mind to the body. It may be that the Time Lords also have the ability to circumvent the limit – in The Five Doctors (1983) the Master is offered a new cycle of regenerations by the High Council in exchange for his help. The fact that the Master is inhabiting a non-Gallifreyan body at the time implies that it is possible to grant them to a non-Gallifreyan, albeit one inhabited by a Time Lord mind (although River Song is shown to have the ability to regenerate due to altered DNA that has similarities to Time Lord DNA, a side effect of being conceived in a TARDIS). In "The Sound of Drums" (2007) the Master is revealed to have been granted a new body by the Time Lords during the Time War with at least one regeneration possible. Non-Gallifreyans are also seen to regenerate in Underworld (1978) and Mawdryn Undead (1983), but with adverse side effects. In Mawdryn Undead these appear to be the result of mishandling stolen technology, but in Underworld they are implied to be the inevitable result of limited technology that reinvigorates, rather than transforms, the subject's appearance (in this case, the Minyans, with whom the Time Lords shared much of their technology), thereby regenerating 'the body, not the soul'.
The BBC's Series 4 FAQ[9] suggests that now the Time Lord social order has been destroyed, the Doctor may be able to circumvent the limit on regenerations; it says: "Now that his people are gone, who knows? Time Lords used to have 13 lives." In Death of the Doctor (a 2010 The Sarah Jane Adventures serial), the Eleventh Doctor responds to a question from Clyde Langer by saying he can regenerate "507" times. Early news reports, before the episode was broadcast, suggested he would say there is no limit to the number of regenerations.[10] Writer Russell T Davies explained in an interview with SFX that the line was not intended to be taken seriously and is instead a commentary. He insisted that the "thirteen lives" rule was too deeply entrenched in the viewer consciousness for his throwaway line to affect it.[11]