Induced changes in resting lung volume (VLR) in the turtle Chrysemys picta (Schneider) had no effect on resting levels of minute ventilation in animals breathing room air but did change their breathing pattern. Increasing VLR caused an increase in the number of breaths in each episode (burst) of breathing but a reduction in the incidence of such breathing bursts and thus an increase in the length of periods of breath-holding. The data indicate that these effects were largely the consequence of

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... hanges in lung volume per se rather than changes in lung gas stores. Although both hypoxia and hypercapnia stimulated ventilation via increases in tidal volume and breathing frequency, they produced distinct changes in breathing pattern. While hypoxia (3% O2) caused an increase in the number of bursts of breathing (B/min) and reduced the number of breaths (b) in each burst (b/B), hypercapnia (5% CO2) increased both B/min and b/B. These data suggest that the size and incidence of bursts of breathing must be under separate control. One consequence of the different effects of hypoxia and hypercapnia on breaths per burst (b/B) was that hypoxic-hypercapnic gas mixtures (3% O2+5% CO2) failed to stimulate ventilation as much as hypercapnia alone. Administration of hypoxic, hypercapnic and hypoxic-hypercapnic gas mixtures to elevate respiratory drive eliminated the effects of changes in VLR on breathing pattern. Thus, although changes in VLR are important in the control of breathholding in animals breathing air, their effect decreases as respiratory drive increases.