[Slainte]

A commonly related story says that the song's title was originally "In The Garden Of Eden" but at one point in the course of rehearsing and recording, singer Doug Ingle became intoxicated and slurred the words, creating the mondegreen that stuck as the title. However, the liner notes on 'the best of' CD compilation state that drummer Ron Bushy was listening to the track through headphones, and couldn't clearly distinguish what Doug Ingle answered when Ron asked him for the title of the song (which was originally "In-The-Garden-Of-Eden"). An alternate explanation, as given in the liner notes of the 1995 re-release of the In-A-Gadda-Da-Vida album, is that Ingle was drunk and/or high when he first told Bushy the title, and Bushy wrote it down. Bushy then showed Ingle what he had written, and the slurred title stuck.

The song features a memorable, "endless, droning minor-key riff,"[3] a guitar and bass ostinato, which is repeated throughout nearly the entire length of the song. It is also used as the basis for extended organ and guitar solos, which are interrupted in the middle by an extended drum solo, one of the first such solos on a rock record and one of the most famous in rock. What made this particular drum solo unique was its surreal tribal sound. Bushy removed the bottom heads from his tom-toms to give them less of a resonant tone, and during the recording process, the drum tracks were subjected to a process known as flanging, producing a slow, swirling sound. It's then followed by Doug Ingle's ethereal polyphonic organ solo (which resembles variations on "God Rest Ye Merry, Gentlemen") to the accompaniment of drums (beginning around 9:20 into the piece). There are then interludes in cut time and a reprise of the original theme and vocals.

Live version

A live version reaching over 19 minutes long was released as part of their 1969 live album. This version, however, has evidence of heavy editing from the actual live recording. The guitar solo, for example, seems to have been recorded in a studio or somewhere else where there was no audience in attendance. The live version also lengthens the drums solo by roughly four minutes and the organ solo by about one minute. The version also omits the bass and drum solo jam (heard from 13:04–15:19 on the studio recording). The version that was edited and released as a single omits the instrumental solos and leaves roughly three minutes of music.

I love this little nugget:

In later years, band members claimed that the track was produced by legendary Long Island producer George "Shadow" Morton, who earlier had supervised the recordings of the band Vanilla Fudge. Morton subsequently stated in several interviews that he had agreed to do so at the behest of Atlantic Records chief Ahmet Ertegun, but he also allowed that he was drinking heavily at the time and that his actual oversight of the recording was minimal.[citation needed] Neither Casale nor Morton receives credit on the album.