For more than three and one-half years a study was made of the amount of locomotor activity of the northern flying squirrel, Glaucomys sabrinus (Shaw). These nocturnal animals were studied in captivity under both natural climatic (outdoor) and controlled artificial (indoor) environmental conditions. Over a sixteen month period, 1285 observations were made under all kinds of weather upon the time at which flying squirrels became active. The average "awakening" time was 35.4 minutes after sunset. Spontaneous locomotor activity, of both individual and groups of flying squirrels, were recorded under natural climatic conditions by means of activity wheels, a treadle-board activity recorder, and an electronic vibration amplifier. Under these outside conditions an inverse relationship exists between the level of activity and the intensity of the nocturnal illumination. Locomotor activity was at a high level between the last and first quarter phases of the moon and declined markedly at full moon, or just afterwards. The relationship appeared to be unaltered by temperature, vapor pressure, the season, or the sex of the animal. The internal physiological "clock" governing this activity cycle persisted even after nine months under constant environmental conditions in which only the light was manipulated to produce 2-3 month periods of continuous light, continuous darkness, or light-dark periods in normal or reversed sequences. In later experiments manipulation of artificial moonlight was used to produce in the animals cycles of activity corresponding to lunar cycles of normal duration and lengthened cycles of 40-days duration or shortened cycles as brief as 7-days in duration. Under 8-hours light and 8-hours darkness (a 16-hour day instead of a 24-hour day), responsiveness differed markedly from the previous experiments i. e. the animals became almost as active under illumination as under darkness. Only after two months under these conditions did daylight activity begin to decline. Whether or not the animals could eventually adapt to a 16-hour day was not determined. This study reveals a series of inherent physiological controls within the animal which govern the level of the spontaneous activity under varying natural climatic conditions and under controlled artificial experimental conditions. The flying squirrel is sensitive to slight changes in light intensity during the dark periods. By observing the responses to varying environmental photoperiods, an attempt was made to understand the inherent rhythms of activity of this species--whether these should be neural, endocrinological, or both. Most nocturnal animals respond mainly to olfactory stimuli. The activity of the flying squirrel is shown by this study to be intimately determined by illumination. / Science, Faculty of / Zoology, Department of / Graduate