Breath-holding induces cardiovascular responses, notably bradycardia and peripheral vasoconstriction, which are known collectively as the diving response. This response is oxygen-conserving, i.e. an augmented response attenuates arterial oxygen desaturation, and is enhanced by apnoea training. To test this hypothesis, we compared heart rate (HR) and arterial oxygen saturation (SaO (2)) in breath-hold divers (BHD) and non-divers (ND). Nine BHD and nine healthy ND performed two static apnoeas (for 30 s and 45 s) and two dynamic apnoeas (for 30 s and 45 s) while swimming underwater at 0.7 m . s (-1). The pool temperature was 26 degrees C. The apnoeas were performed at 60 % of forced vital capacity. Heart rate (HR) and SaO (2) were recorded before breath-holding and at its end and are expressed in % change from rest values (DeltaHR and DeltaSaO (2)). Comparisons between BHD and ND showed that DeltaSaO (2) were lower in divers after both static apnoeas for 30 s and 45 s (- 2.8 % vs. - 5.5 %; - 3.2 % vs. 6.3 %; p < 0.05, respectively) and dynamic apnoeas (- 6 % vs. - 10.1 %; - 7.2 % vs. - 12.3 %; p < 0.05, respectively). The change in HR did not differ between the two groups and negative linear relations were found between DeltaHR and DeltaSaO (2) in both divers and ND (r = 0.66 and 0.61, respectively; p < 0.001). Moreover, the slope was lower for the divers (- 0.785 vs. - 0.1429; p < 0.001). Our results suggest that apnoea training explained the greater oxygen conservation seen in the divers in both static and dynamic conditions.