Hartnell's deteriorating health (he suffered from undiagnosed arteriosclerosis) began to affect his ability to learn his lines, with the problem increasing as his time on the series progressed. In addition, he had a poor relationship with a new production team on the series following the departure of Lambert. He left "Doctor Who" in 1966. When he departed the producer of the show came up with the idea that, since the Doctor is an alien, he could transform himself physically, thereby renewing himself. Hartnell suggested the actor who should play the new Doctor, stating that "There's only one man in England who can take over, and that's Patrick Troughton." In the fourth episode of the serial "The Tenth Planet" the First Doctor regenerates into Troughton's Second Doctor.