On October 6, while recording Eight Days a Week, John was working out the guitar riff that would become the basis of I Feel Fine, a song they recorded only 12 days later. "I actually wrote I Feel Fine around the riff which is going on in the background," John said in December 1964. "I tried to get that effect into every song on the LP, but the others wouldn't have it.

"I told them that I'd write a song specially for this riff, so they said, 'Yes. You go away and do that,' knowing that we'd almost finished the album. Anyway, going into the studio one morning I said to Ringo, 'I've written this song but it's lousy', but we tried it, complete with riff, and it sounded like an A side, so we decided to release it just like that'."

Apart from the riff, the distinctive feature of I Feel Fine is the sound of feedback which slides into the opening chords. It was one of those discoveries which they made in the studio and decided to use on a track because they liked it.

John's guitar was leaning against an amplifier after a take and set up an electronic whine. This marked a subtle shift in their approach to recording. Having mastered the studio basics, they started encouraging George Martin to take them further, finding fresh sources of inspiration in noises previously eliminated as mistakes (electronic goofs, twisted tapes, talkback). Feedback was to become a familiar part of recording — used by artists such as Jimi Hendrix and The Who — but John remained proud of the fact that the Beatles were the first group to actually put it on vinyl.

I Feel Fine, John's most optimistic song to date, became a No 1 single in both Britain and America.