We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold.

I remember saying something like:

Raoul Duke: I feel a bit lightheaded. Maybe you should drive.

 Suddenly, there was a terrible roar all around us, and the sky was full with what looked like huge bats, all swooping and screeching and diving around the car, and a voice was screaming:

Narrator and Raoul Duke (simultaneously): Holy Jesus! What are these goddamn animals?!

Dr. Gonzo: Did you say something?

Raoul Duke: Hm? Never mind. It's your turn to drive.

 No point in mentioning these bats, I thought. Poor bastard will see them soon enough.

Well worth a read also amazing illustrations by Ralph steadman.