Mystery Hills star may take Speidi's place on "i'm a celebrity get me out of here"

They may or may not be celebrities. They're definitely out of there.

Not even the promise of three weeks' worth of prime-time self-promotion was enough to keep Heidi and Spencer Pratt from jumping ship on I'm a Celebrity...Get Me Out of Here!

Though their first attempt at quitting NBC's jungle-set reality competition didn't stick, Speidi has, for the second and ostensibly final time, vowed to abscond from Costa Rica after being unable to shampoo rough it.

All told, the duo's run will last just two episodes. They will return for the live portion of tonight's show before bowing out. Sources confirm to E! News that the Pratts are scheduled to return to Los Angeles later tonight, immediately after the taping.

"They're leaving right after the live portion of the show," an NBC source said. "The fact that they are leaving the show and deserting their charities is lame."

Meanwhile, E! News has exclusively learned that I'm a Celebrity... producers are trying to replace the rapping, breakdown-prone couple with another member of The Hills cast, who would continue playing for the couple's chosen charity.

The identity of the replacement remains hush-hush at this point.

I'm a Celebrity...Get Me Out of Here! runs Monday through Thursday on NBC, with the finale set for June 24.