APPROXIMATELY 8 HOURS INTO HIS DUTY DAY, AFTER FLYING FOR APPROXIMATELY 6 HOURS. THE FIRST OFFICER EXPERIENCED A 20 SECOND EPISODE OF SIMPLE VASO-VASO SYNCOPE (FAINTING), AS DIAGNOSED BY THE FAA AEROMEDICAL STANDARDS DIVISION MANAGER. THIS OCCURRED AS HE WAS ENROUTE TO THE LAVATORY FROM THE RIGHT SEAT. HE WAS REVIVED WITH AN AMMONIA INHALENT INTERVIEWS WITH THE FIRST OFFICER BY AVIATION MEDICAL EXAMINERS REVEALED THAT HE HAD BEEN ILL WITH INFLUENZA FOR THREE WEEKS PRIOR TO THE INCIDENT BUT THOUGHT HE HAD RECOVERED. IN ADDITION, DURING THIS THRE WEEK PERIOD HE WAS INTENTIONALLY DIETING AND EXPERIENCED A TEN POUND WEIGHT LOSS. THE AEROMEDICAL STANDARDS MANAGER BELIEVERS THAT THE EPISODE OF SYNCOPE IS NOT LIKELY TO OCCUR AGAIN. THE FIRST CLASS MEDICAL CERTIFICATE WAS SUBSEQUENTLY REISSUED TO THE FIRST OFFICER WITH THE AUTHORIZATION OF THE FEDERAL AIR SURGEON.