Studio Two, EMI Studios, London
The final overdub onto what would be titled "I Want To Tell You" was of Paul's bass guitar. (Recording the bass separately onto a vacant track of the four-track tape allowed greater manipulation of its sound during mixing.) Four mono mixes of this song and five of "Yellow Submarine" concluded this 7:00 pm to 2:30 am session.
Source: The Complete Beatles Chronicle - Mark Lewisohn