I was flying an MD80 over the Sierras at FL350 one night and encountered a mountain wave. I got the rising wave so the autothrottle went to idle but was still going to exceed mach limit so I used full speed brakes to maintain altitude. Next came the descending wave and I retracted the speed brake and the autothrottle went to max cruise thrust. We were still losing airspeed so went to max continuous thrust. We were just able to maintain our speed above low speed buffet so didn't have to descend with no clearance. The changes were smooth but rapid, not like windsheer where the airspeed change is abrupt. There was no turbulence during this encounter.