The session took place in studio three at EMI Studios, Abbey Road, and lasted from 8pm-1.15am. At this time the song had the working title Mark I.
Although quite different from the final version of Tomorrow Never Knows, Mark I represented a huge leap forward in recording terms for The Beatles. Take one, which was released on 1996's Anthology 2, sounded quite unlike anything the group had done before - and, indeed, bore little resemblance to contemporary music in general.
6 April 1966 was primarily spent on the song's rhythm track and vocals. The basis of the final version of Tomorrow Never Knows was take three, the last of the day's attempts.
The rhythm track contained Ringo Starr's drums, Paul McCartney's bass guitar and George Harrison's tambura. Lennon then added his lead vocals, which were fed through a Hammond organ's Leslie speaker during the second half.
It meant actually breaking into the circuitry. I remember the surprise on our faces when the voice came out of the speaker. It was just one of sheer amazement. After that they wanted everything shoved through the Leslie: pianos, guitars, drums, vocals, you name it!
The Complete Beatles Recording Sessions, Mark Lewisohn
The song's various tape loops and effects were added on 7 April, and a final overdub containing more vocals, guitar, organ, tambourine and piano was made on 22 April.