In certain idiopathic cases of narcolepsy, who are moderately obese, the arterial pC02 is found to have risen to the levels usually observed in normal sleeping subjects. As in the Pickwickian Syndrome, it is also possible to explain the episodes of sleep in narcolepsy in terms of the narcotizing effect of hypercapnia. On the other hand, the fact that narcoleptic persons are continually in a state of drowsiness in the electroencephalographic sense, brings to mind the possibility that hypercapnia may not be the cause, but the effect of narcolepsy.