"Fear Factor" producers' plans to serve fresh glasses of donkey semen to contestants on the next episode had NBC execs so concerned ... they gave serious thought to killing the stunt, TMZ has learned.

Sources involved in the production tell us the stomach-churning stunt was shot last summer -- but NBC honchos were having a tough time swallowing this one as the air date approached.

We're told the challenge involved teams of twins drinking the full glass of donkey semen -- with a glass of urine thrown in for good measure. Contestants had to drain both glasses in order to move on to the next round.

Our sources say NBC execs had multiple pow-wows in the months after the stunt was shot ... but eventually gave FF producers the thumbs up.

Calls to NBC were not immediately returned.

The episode airs this coming Monday -- and yes, we're told multiple contestants actually do (gulp) drink up.

So that's what they mean by donkey punch.



