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1. Breathing hypoxic gas through an external dead space (ca. 1200 c.c.) stimulated ventilation disproportionately. A loop (ca. 250 c.c.) in the inspiratory pathway reduced the effect.

2. The alveolar time patterns of P_CO2 and P_O2 characteristic of tube breathing with or without the loop have been simulated in moderate hypoxia by changing the composition of inspired gas at selected intervals after the beginning of inspiration.

3. Supplying CO₂-free gas in late inspiration usually stimulated ventilation, but less than did real tube breathing. Supplying CO₂-free gas early in inspiration usually depressed ventilation. The difference between the `CO<sub>2</sub>-free late' and `CO₂-free early' effects was 20% of the control ventilation (P < 0·001), i.e. was nearly the same as between the effects of real tube breathing without and with the loop.

4. Tube-like P_{A, O2} time patterns had no effects.

5. A—a P_CO2 and P_O2 gradients remained constant throughout.

6. The _E, f and V_T relations were unaltered in tube breathing.

7. The respiratory system can discriminate between small differences in time patterns of P_{A, CO2} but not of P_{A, O2}; the signal is amplified by steady hypoxia. The arterial chemoreceptors are probably responsible for these effects.

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