A study of the platelet count in rats exposed to normobaric hypoxia for 4 weeks revealed that hypoxia alone, without any change in barometric pressure, considerably reduced the number of thrombocytes. In order to elucidate the mechanism of hypoxic thrombocytopenia, the survival and recovery of 51Cr-labelled platelets were tested. The survival and recovery of platelets from hypoxic rats injected into normoxic recipients were normal. However, when 51Cr-labelled platelets from normoxic rats were injected into a hypoxic recipient, the survival was found to be normal but the recovery of those platelets was lower than in the normal rat. This finding suggests that the reduction of platelet counts in normobaric hypoxia may be due to the platelets’ partial sequestration.