The alterations of blood gases, pH, electrolytes and haemoglobin during 45 min storage in ice-water were measured in 6 types of syringes (1 glass and 5 plastic syringes, among these 3 "blood gas samplers"). It was confirmed that pO2 generally is not stable in plastic syringes. However, considerable differences among plastic syringes were found in this respect, the smallest increase occurring in an ordinary 2 ml syringe for injections and the greatest in one of the special blood gas samplers. Due to the "buffering effect" of deoxyhaemoglobin, the alterations of pO2 are smaller in the hypoxaemic than in the normoxaemic range. Relevant pO2 alterations in plastic syringes are demonstrable after 20 minutes. It is concluded that blood collected in plastic syringes must be analysed within 15 min after sampling, otherwise glass syringes should be used for blood collection. Deviations of pCO2, pH and electrolytes are described in detail. In general, they are due to sampling rather than to storage, and can be effectively minimized by a small dead space of the syringe and by use of an electrolyte-balanced heparin solution. The danger of erroneous haemoglobin measurements due to unequal resuspension of the red cells after storage is pointed out.