A young man had two dangerous episodes of transient loss of consciousness during apnea diving in a swimming pool. Medical and neurologic examination results were normal. Standard autonomic test results (including heart rate variability, baroreflex sensitivity, tilt-table test, and Valsalva ratio) were unremarkable, with the exception of an increased blood pressure decrease during early phase II of the Valsalva maneuver. Syncope with arrhythmic myoclonic jerks could be evoked by a strong straining maneuver. Simultaneous physiologic recordings showed extreme blood pressure and cerebral blood flow velocity decreases and electroencephalographic slowing during syncope. The electrocardiogram showed a continuous sinus rhythm with a progressive tachycardia. The authors' findings were not compatible with baroreflex failure or vasovagal mechanisms (Bezold-Jarisch reflex activation) as the underlying causes. The authors concluded that mechanical factors (strong reduction of blood reflux to the heart) in combination with a reduced threshold of the brain for developing ischemia-related arrhythmic myoclonic jerks were responsible for Valsalva-induced syncope in the patient.