The effect of immobilization in bed on metabolism and function of human blood platelet was studied. Blood platelets taken from patients with bone fractures after long term bed rest (14 days and 28 days) demonstrated significantly reduced concentration of total adenine nucleotides (after 28 days reduction about 30%). This decrease of total platelet adenine nucleotides af er immobilization in bed is probably caused by s imulation of platelet secretory process. Thrombin which released from confrol platele s 58.2%±1.5% of total adenine nucleotides liberated decreased amounfs (only 23.1 %±3.3% of total) of nucleotides from patient platelets isolated after 28 days of immobilization in bed. Loss of nucleotides from platelets was accompanied by sligh ly increased extent of platelet aggregation. It is concluded that during bed rest the reactivity of blood platelets (aggregation and release reaction) is stimulated.