The bullfrog was chosen for study because its labyrinth is very similar to that of humans. Since it is an amphibian, preflight surgery could be performed above water, but it could be kept in water during the flight. The water medium served to cushion the vibration and acceleration of launch and to facilitate gas exchange with the organism. Both flight frogs had electrocardiogram (ECG) electrodes implanted in their thoracic cavities and microelectrodes implanted in their vestibular nerves. The frogs were demotorized to prevent them from dislodging their implanted electrodes and to reduce their metabolic rates. With this lowered metabolic activity, the frogs could survive in good health without being fed for as long as one month. Immersion in water allowed the frogs to breathe through their skin. The water medium also helped to move carbon dioxide and heat away from the animals. Surgical preparation of the flight frogs was completed about 12 hours before launch, and the animals were sealed inside the FOEP. A backup FOEP was also prepared with similar specimens. The flight FOEP was installed in the satellite about three hours before launch. The centrifuge was activated as soon as possible once the satellite was in orbit and stabilized at 10-3 g. The centrifuge applied gravity stimuli in cycles. Each cycle lasted about 8 minutes and consisted of the following: a 1-minute period without acceleration, an 8-second period when rotation slowly began, 14 seconds of constant 0.6 g, an 8-second period when rotation slowly stopped and a 6-minute period when aftereffects of rotation could be measured. Cycles were performed every 30 minutes during the initial 3 hours in orbit and less frequently during the rest of the flight. The OFO experiment continued until the seventh day in orbit, at which time the on board battery failed. Recovery of the OFO spacecraft and FOEP hardware were not required. The experiment was successful. ECG indices showed the flight frogs to be in good health during the entire flight. Vestibular recordings were made as expected. Two equipment malfunctions occurred during the flight: pressure in the canister increased to 11 psi, and the temperature decreased to 55 degrees F for nine hours. However, control experiments performed on the ground showed that these malfunctions had little effect on the outcome of the flight experiment. Several vestibular response changes were noted during the early period in weightlessness. All of the observed changes reverted to normal during the last 10 to 20 hours of the flight, suggest adaptation.