Preliminary investigations into the physiological effects of breathing retraining techniques in order to establish the feasibility of the proposed methodology, produce data for power calculations and determine the potential for future research in this area.Two preliminary, exploratory studies with an experimental design.Research laboratories.Study 1: 15 patients with mild to moderate asthma and nine healthy volunteers. Study 2: 15 healthy volunteers.Physiological data from asthma patients and healthy volunteers were recorded before, during and after a single session of slow breathing and breath holds. A separate group of healthy participants carried out the slow breathing technique with and without visual biofeedback.The primary outcome was end-tidal carbon dioxide (ETCO 2 ), and the secondary outcomes were respiratory rate, pulse rate, oxygen saturation and lung function.All groups showed an increase in ETCO 2 from baseline during slow breathing. Study 1 found a mean rise in ETCO 2 of 0.48kPa [95% confidence interval (CI) 0.28–0.68] for healthy participants and 0.46kPa (95% CI 0.29–0.63) for asthma patients. In healthy volunteers, ETCO 2 stayed above baseline for 5minutes after the intervention. ETCO 2 rose minimally (mean 0.06kPa) in both groups after breath holds. Study 2 found a mean rise in ETCO 2 of 0.35kPa (95% CI 0.09–0.60) during slow breathing with visual feedback, and 0.36kPa (95% CI 0.13–0.60) during slow breathing without visual feedback.The results of these preliminary studies provide data to power larger studies. They suggest that ETCO 2 rises during slow breathing in both asthma patients and healthy volunteers, and that this effect may persist beyond the intervention itself in healthy volunteers. The use of visual biofeedback had no effect on ETCO 2 in healthy volunteers.