Brad Pitt submitted to a drug and alcohol test early on during the investigation into allegations he abused his son ... but it was not something he was forced to do.
Sources close to investigators tell TMZ ... the test went down within a couple days of the alleged incident on the plane. We're told Pitt voluntarily provided a urine sample in an attempt to show he had nothing to hide, and to clear up any "exaggerations."
As we first reported ... both sides agree he drank heavily on the plane, but Angelina Jolie's people say it was after the alleged incident with Maddox -- Brad's camp says it was before.
Obviously, the test -- first reported "In Touch" -- wouldn't show when Pitt had been drinking, but we're told he wanted the test to blunt any claims he's abusing drugs or alcohol. DCFS did not require it for its investigation ... Pitt simply offered.