Chris Kattan -- Out of It On Airplane Before DUI Arrest

Chris Kattan was so out of it on an L.A. bound jet ... paramedics were called to help him off the plane when it landed in L.A. -- and that was just hours before he was busted for DUI.

Law enforcement sources tell TMZ ... late Sunday afternoon ... the flight crew radioed that Kattan was acting in a bizarre fashion ... walking up and down the first class aisle, talking to himself about running out of medication.

We're told Chris was ranting that the flight had been delayed 3 hours and his Rx supply had run out.

Sources say 2 doctors who happened to be on the plane calmed Chris down and when the plane landed paramedics from the L.A. City Fire Dept. helped him off the plane and talked to him. We're told Chris got some sort of treatment and then left on his own.

Hours later ... Chris was on the 101 Freeway ... driving erratically before slamming into a parked Dept. of Transportation vehicle inside a coned area. He flunked the field sobriety test -- which is apparent from the video -- and was arrested for DUI.