"Endorphins," wow, how could I
live without 'em?
I always feel so fantastic from them I could kiss a mule (well, naw!)
I am no rocket scientist, but I feel
something extraordinary, not only during the ride, but sometimes for hours afterwards. In fact, I feel an overall buoyant strength and energy keeping me "afloat" most of the time, especially for the first hour or so after cycling (and during, of course.) Then again, there's
tired... 
The best test for whether you have endorphins going for you or not is to get in a car and drive for about an hour.
Doesn't feel quite the same, does it? In fact, it feels kind of the opposite...first a heaviness, then boredom, followed by a gradually increasing anxiety and impatience...