CABIN WARNING HORN ALERTED FLC THAT THE ACFT WAS DEPRESSURIZING. AT FIRST; THE SO WAS UNABLE TO GAIN CTL OF THE PRESSURE WITH THE OUTFLOW VALVE AND THE CAPT DECLARED AN EMER AND COMMENCED A DSCNT. EVENTUALLY THE SO REGAINED CTL OF THE OUTFLOW VALVE AND THE FLC WAS ABLE TO CLB BACK TO ALT AND CONTINUE TO DEST.

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DURING CRUISE AT FL330 CABIN ALT WARNING HORN SOUNDED. CABIN ALT WAS SLIGHTLY ABOVE 10000 FT. WE DONNED THE OXYGEN MASKS AND I REQUESTED A DSCNT TO FL250. THE SO SILENCED THE HORN; CHKED THE AIR CONDITIONING/PRESSURIZATION SYS AND RAN THE LOSS OF PRESSURIZATION CHKLIST. DURING THE DSCNT; THE HORN SOUNDED AGAIN AS CABIN PRESSURE DECREASED TO 10000 FT. I ASKED THE SO AGAIN IF HE WAS ABLE TO CTL THE PRESSURIZATION. HE WAS NOT SURE; STATING WE MIGHT HAVE A HOLE IN THE ACFT. SINCE CTL OF PRESSURIZATION WAS QUESTIONABLE; I DECLARED AN EMER DUE TO PRESSURIZATION PROBS AND REQUESTED A DSCNT TO 10000 FT. WE ACCOMPLISHED THE EMER DSCNT CHKLIST. DURING DSCNT WE REGAINED CTL OF PRESSURIZATION WHEN SO SELECTED ANOTHER MODE OF CTLING THE OUTFLOW VALVE WHICH HAD ALWAYS APPEARED CLOSED. WE REQUESTED AND RECEIVED CLRNC TO LEVEL AT 16000 FT. THE SYS FUNCTIONED NORMALLY IN THE NEW MODE. WE TERMINATED THE EMER; REQUESTED FL230 AND CONTINUED TO OUR DEST AT THAT ALT. WE ADVISED OUR DISPATCHER OF THE SIT AND RECEIVED WX; FUEL BURN AND TIME TO DEST. THE FLT ATTENDANTS WERE KEPT INFORMED OF OUR STATUS. THE PAX NEVER KNEW OF ANY PROBS SO WE DID NOT ALARM THEM BY MAKING A PA. THE CREW WORKED VERY WELL TOGETHER AND WERE VERY COORDINATED. THE ONE AREA OF IMPROVEMENT WOULD BE WITH THE SO. IF HE HAD CLOSED THE OUTFLOW VALVE SOONER; WE WOULD NOT HAVE PERFORMED THE EMER DSCNT.