1. We have studied the reflex respiratory responses to two-breath alternations of fractional inspired oxygen (FI,O2) in normoxic (reared in room air) and chronically hypoxic kittens (born into and reared in an FI,O2 of 0.13-0.15) on post-natal days 1, 2-3, 4-8 and 9-14. 2. Respiration was measured non-invasively in the unanaesthetized kittens. Test runs (with alternations of FI,O2 between 0.21 and 0.14) and control runs (with an FI,O2 of 0.21) were carried out during quiet sleep. 3. The degree of alternation in tidal volume, inspiratory time, expiratory time, frequency, drive, timing and instantaneous ventilation components of the respiratory response was compared during control and test runs. 4. There was little response to control runs in either group at any post-natal age. 5. In normoxic kittens we found no significant reflex response in any respiratory variable to test runs before day 4. However significant alternation was found in expiratory time, frequency and ventilation at days 4-8 and in tidal volume at days 9-14. 6. In chronically hypoxic kittens there were no significant differences between control and test runs at any of the ages studied. 7. In the normoxic group increases in the response with post-natal age probably reflect post-natal increases in the sensitivity of the peripheral chemoreceptors. The lack of development in the chronically hypoxic group may indicate abnormal function or delayed maturation of the peripheral chemoreceptor sensitivity to hypoxia. 8. The results suggest that the method can be used to detect developmental and pathological changes in the arterial chemoreflex.
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