A brief review of the literature on the effects of darkness and radiation is given, followed by a report of the figures obtained in red, white and platelet counts and percentage haemoglobin, during short and long periods of darkness. Tables and graphs showing the changes occurring are included. The outstanding features of a short period of darkness (15-28 days) are an initial decrease in reds, platelets and haemoglobin, and an initial increase in whites, with a return to normal values before the end of the period. Total solids are low, and a slight acidosis is indicated. The outline of events during a long period (9 months) of darkness shows first the same changes as those occurring at the beginning of a short period, followed by increasing red counts and haemoglobin percentage for 3 months. White and platelet counts decrease. At the end of 8 months, there occurred lowered red, platelet and haemoglobin values, and a marked increase in white count, due to acute illness caused apparently by the darkness. The return to light after both short and long periods of darkness produced temporary increases in reds, platelets and percentage haemoglobin.