Jon Pertwee (1919-1996) from '70-'74. Regenerated due to the fact that he was poisoned oleh radiation on the planet Metabelis III. 'Planet of the Spiders' is absolutely wonderful. I have watched it and would definitely recommend it to any Whovian.
Portrayed oleh the phenomenal William Hartnell (1908-1975) from '63-'66. This Doctor regenerated in the virtue of a strenuous battle with the Cybermen, 'The Tenth Planet'. :)
