Supposedly it was nitrous oxide, although the story goes that due to Dennis Hopper’s past drug use, David Lynch had wanted Hopper to inhale helium. Hopper opted for nitrous, but wasn’t aware that the gas would have such a pronounced effect on his performance during filming. Evidently, the footage stayed in the final cut. Creepy!

Wow, that only strengthens my theory about Dennis Hopper as a ‘the real deal’ kind of actor.