Competitive underwater hockey play demands frequent and prolonged breath-hold dives. To see whether participants were physiologically adapted to breath-hold diving we studied the ventilatory response to carbon dioxide (CO2) and the breath-hold times of 34 male, underwater hockey players (divers) and compared them to 28 male, dry-land sportsmen (athletes). The divers showed an increased tolerance to CO2, the mean (+/- SD) slope of the CO2 response curve being 1.08 (+/- 0.55) liter.min-1.mmHg CO2(-1) when measured by the rebreathing method. This was significantly less (P less than 0.005) then that of the athletes 1.68 (+/- 0.72) liter.min-1.mmHg CO2(-1). The breath-hold times measured at 2 lung volumes did not differ significantly between the 2 groups. A subgroup of 8 international underwater hockey players exhibited prolonged breath-hold times but were otherwise similar to the rest of the divers in the other measured parameters.