An investigation of the importance of water contact, cooling, and apnea to the development of the diving reflex in humans was performed in two experimental series. The first series involved exposure of different parts of the face to direct water contact, while the second consisted of face immersion in water baths of varying temperatures. Data were obtained on heart rate (ECG's) and peripheral vasoconstriction (by indirect blood pressure using sphygmomanometry). Results were interpreted to indicate that the diving reflex is actuated by both facial cooling and apnea, but not by any direct effect of water contact other than cooling.