C is frequently used in the treatment of apnea of prematurity and stimulates ventilation by a central mechanism in adults. However, the ventilatory effect of C is significantly reduced in chronically carotid body denervated lambs (J. Appl. Physiol, 61:133-7,1986). To test the hypothesis that C alters the chemosensory discharge of the CSN, we studied 9 kittens (17 to 21 day old) and 6 adult cats anesthetised, artificially ventilated and prepared for the recording of CSN chemosensory discharge. They received 4 doses of C of 10 mg/kg each at an interval of 20 min, either as an i.v. bolus(6 kittens, 3 cats) or as constant infusion over 20 min (3 kittens, 3 cats). Arterial blood pressure (ABP) and the activity of a few CSN chemosensory fibers were recorded before and after C. Plasma C levels were measured 15 min after injections by HPLC. In kittens, each bolus injection of C induced a transient increase in CSN discharge and a slight decrease in ABP. The 1st dose increased CSN from 4.2 ± 0.6 to 8.1 ± 0.9 imp/sec (p < 0.001) and decreased ABP from 49.5 ± 5.0 to 38.0 ± 3.0 mmHg, (p < 0.01) in kittens. Both CSN and ABP were back to control at 5 min after each dose of C despite the incremental increase in plasma C levels. Similar responses were observed in adult cats. Constant C infusion influenced neither CSN nor ABP. Conclusion: C transiently increases the CSN chemosensory discharge, an effect which was associated with a decrease in ABP and was observed only after bolus administration. It is suggested that the early and transient increase in CSN is related to an indirect effect of C, e.g., on carotid body perfusion, but can not account for the sustained stimulation of ventilation induced by C. (Funded by MRC: MT-12741 and APQ)