On October 3, 1996, at 2037 mountain daylight time, a Beech 1900D, N154ZV, operated by Mesa Air Group, as Air Shuttle flight 7632, was involved in an incident involving partial crew incapacitation while on a flight from Denver, Colorado, to Pueblo, Colorado. There was no damage to the aircraft and no injuries to the two crew or 14 passengers. The flight was operating as a scheduled domestic passenger flight under Title 14 CFR Part 135. Visual meteorological conditions prevailed and an IFR flight plan was filed. The crew made a normal approach and landing at Pueblo after going on oxygen. The two crew members went on oxygen when they felt dizzy while in cruise flight at 13,000 feet msl. They stated that the oxygen use made them feel better immediately. One passenger complained of possible dizziness when interviewed by the HAZMAT team following arrival. No fumes or smoke were discernible by either the crew or passengers. The flight had been airborne for approximately 30 minutes when the event occurred. A check with local, state, and federal officials covering the area between Colorado Springs, and Pueblo, was conducted with negative results and no other flights in the area at the time reported any abnormalities. The voice recorder transcript (copy attached) provided support for the pilots' report of dizziness and disorientation. In addition, crew conversation, as recorded, provided information that the pilots discussed their lack of a recent meal. However, a review of the pilots' activities for the 72 hours prior to the incident provided information of normal meal habits and provided no information of common food intake or other activities. The Pueblo Fire Department responded with a HAZMAT team which conducted air sampling and inspected the luggage and cargo with negative results. Medical examination and partial toxicological test following the incident provided no evidence to explain the experience. (Copies attached) Examination of the aircraft was conducted with negative results. The aircraft had been involved in an accident on May 2, 1996, during which the right main landing gear collapsed. (FTW 96-F-A198) No information was found in reviewing that accident and the repairs conducted that would contribute to this incident. (The Safety Board IIC was the same person as on this incident). A test flight was conducted on October 5, 1996 using the incident aircraft and crew and the services of an industrial hygienist. The altitude of the incident flight and route were used and no information was found to explain the event. A copy of the hygienist's report is attached. At the request of the Safety Board IIC, the aircraft and crew were tracked by the Director of Safety for Mesa Airlines from return to service through the end of 1996. No reports of any aircraft abnormalities have occurred, and neither pilot has experienced any difficulties since the incident.