An MD80 was unable to maintain pressurization at FL340 with engine and airfoil anti-ice turned on. The flight crew declared an emergency and descended until pressurization capability was restored.

Narrative

We were at cruise; FL340; wing and engine anti-ice on. No ice visible. After about 40 minutes flight time; the CABIN ALT warning light and aural alert were received. Cabin altitude was at 10;000 FT and climbing at 500 FPM. Checked pack pressure/valves - all normal. Outflow valve fully closed. Pressurization controller appeared to be functioning normally. It was obvious we had a leak of some kind. Requested descent from ATC but they were not responsive; so we declared emergency; squawked 7700; and began an emergency descent. During the descent we completed the Cabin Alt Warning checklist. When I idled throttles the cabin started a rapid climb; so as we got into the low 20's I opened throttles to try to stabilize the pressure. Cabin then began to recover; so we restored automatic outflow valve per checklist. Pressurization appeared to operate normally for rest of the flight. Cabin did not climb higher than about 11;500 FT; and passenger masks were not needed nor deployed. No passengers seemed to be aware of the pressure loss. We briefed the cabin crew; and completed the flight to our destination; which seemed to be the most reasonable course of action.We had a long 14 hour day previously due to other mechanical issues; followed by a short night in a hotel full of noisy teenagers; and an early start. Neither of us had a good night's sleep. I think the Captain handled it well in spite of our fatigue; the adrenalin kicks in.