SUMMARY: 1. An investigation of the diurnal variations of circulating leucocytes has been carried out at Spitsbergen under constant conditions of diet, activity and lighting. 2. The effect of reversal of the activity-sleep pattern on the diurnal variations was studied, and the reversal of lymphocyte, eosinophil and basophil periodicity occurred in less than 3 days. The neutrophil periodicity was reversed in from 3 to 6 days. 3. The periodicity is ultimately dependent upon habit and activity. 4. The neutrophil variations appear to be controlled by a mechanism different from that of the other leucocytes. 5. The diurnal variation in adrenocortical activity is assumed to have reversed in response to the activity-sleep reversal.