Temperatures ranging from room temperature (20 degrees C) to 42 degrees C are generally not considered to have an activating effect on platelets. However, this assumption is not supported by clinical phenomena that result in hemostatic failure related to hypothermia. In this study, we investigated the effect of temperatures between room temperature (20 degrees C) and 42 degrees C on human blood platelets and found that room temperature causes marked activation of platelets. Major changes in platelet morphology were seen at 20 degrees C compared to resting platelets at 37 degrees C. Platelet morphology was investigated with noninvasive live cell techniques (light microscopy and dynamic and static light scattering), as well as with transmission and scanning electron microscopy. The changes in platelet morphology correlated with the expression of the activation marker, activated glycoprotein (GP) IIb-IIIa, measured by flow cytometry. Twenty-five percent to 30% of platelets expressed activated GPIIb-IIIa after exposure to 20 degrees C for 10 minutes. In the presence of serotonin re-uptake inhibitors, the serotonin content of platelets at 20 degrees C was twice that of resting platelets. In comparison, moderate heat shock conditions (42 degrees C for 10 minutes) caused no signs of platelet activation as indicated by the absence of morphological alterations, no expression of activated GPIIb-IIIa, and no changes in serotonin content. These results show that room temperature by itself significantly activates platelets and has an effect on the platelet serotonin content. This may contribute to both the functional lesion associated with 22 degrees C storage of platelets for transfusion and the in vivo hemostatic failure after hypothermia.