A little background, courtesy of Wikipedia:

The original 1967 version of “Ain’t No Mountain High Enough” was a top twenty hit. According to record producers, Terrell was a little nervous and intimidated during recording because she hadn’t rehearsed the lyrics. Terrell recorded her vocals alone with producers Harvey Fuqua and Johnny Bristol, who added Gaye’s vocal at a later date.[1] “Ain’t No Mountain” peaked at number nineteen on the Billboard pop charts, and went to number three on the R&B charts.

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