Relative contributions of chemical and non-chemical respiratory stimulations to breath-holding time (BHT) were examined in assisted (Funado) and unassisted (Kachido) breath-hold divers (Ama). In the Funado the magnitude of the chemical contribution was reduced, though statistically not significant. On the other hand, in the Kachido no difference in chemical contribution was seen from the control. This was considered to be due to the fact that ventilatory response to CO2 was reduced in the Funado, but not in the Kachido. Despite the decreased contribution of CO2 drive to BHT, absolute BHT in the Funado was not prolonged. This may be related to sensitization of the respiratory centers to non-chemical stimulation. Such adaptation would be effective for preventing the danger of losing consciousness in the Funado who face extreme hypoxia on returning to the surface from a dive.
