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1. The responses of aortic chemoreceptors and pulmonary stretch receptors of cats were studied by recording impulses in individual fibres at normal body temperatures and thereafter at various temperatures down to 24-26° C while cooling the cat with ice.

2. Reduction of metabolism by lowering the temperature did not obviously slow the P_O2 sensing mechanism of chemoreceptors but it greatly slowed the development of excitation after circulatory arrest. It also greatly reduced the excitatory effect of hypoxia.

3. The Q₁₀ for the frequency of discharge of chemoreceptors (during maximal activity) was estimated by comparing the activity of the endings at normal body temperature after circulatory arrest (i.e. at a local P_O2 of 0 mm Hg) with similar activity at reduced body temperatures. This averaged 2·5 in nine endings. The Q₁₀ for the peak frequency of discharge also averaged 2·5 in seven endings. These values are similar to those of some mechano-receptors.

4. Apart from the reduction in the frequency of discharge (Q₁₀ = 2·6) lowering the temperature did not alter the stimulus—response relationship of pulmonary stretch receptors.

5. The poor responses to ACh at lower temperatures indicate that ACh is not likely to be a transmitter at chemoreceptors.