1. Carotid body blood flow (c.b.f.), the arterio—venous oxygen (A—V O2) difference and oxygen consumption were measured in forty-seven cats, anaesthetized with pentobarbitone, paralysed with gallamine and ventilated artificially. Carotid sinus and cervical sympathetic nerves were intact throughout.

2. A system for perfusing the carotid body artificially with blood is described and evidence is given which shows that similar results were obtained whether the carotid body was naturally or artificially perfused.

3. With arterial pressure, blood gas tensions and pH within physiological limits, c.b.f. varied between 33 and 68 μl./min, average 41·5; A—V O2 difference between 0·21 and 0·46 ml./100 ml., average 0·34, and calculated oxygen consumption between 0·115 and 0·195 μl. O2/min, average 0·147.

4. With constant mean arterial pressure, hypoxia (30–40 mm Hg Pa, O2) or hypercapnia (> 50 mm Hg Pa, CO2) resulted in a small increase of c.b.f., up to 14 μl./min above control; an average fall of A—V O2 difference by 49% of control and an average fall of oxygen consumption by 36% of control.

5. Carotid body blood flow fell linearly with mean arterial pressure over the range 100–170 mm Hg, the slope of the curve varying between 0·78 and 1·22 μl. min−1. mm Hg−1. M.A.P. A—V O2 difference was unaffected so that oxygen consumption fell in proportion to c.b.f.

6. It is concluded that the unique response of the carotid body to these stimuli is a fall in oxygen consumption and that this bears a closer relation to the known pattern of chemoreceptor discharge than do changes in total blood flow.