The influence of temperature and duration of storage on glutathione peroxidase (GSH-Px) activity in plasma and whole blood samples of horses was investigated. Furthermore the relationships between the GSH-Px activities in plasma and whole blood as well as the GSH-Px values related to hematocrit and the hemoglobin content of the blood samples of 93 different horses were calculated. At 20-22 degrees C, the GSH-Px activity in plasma and whole blood samples remained stable over at least three days while it was reduced by 38% and 65% after 2 and 3 days, if the whole blood samples were kept stored after being diluted (lysate). The GSH-Px activity was stable over three days in plasma, whole blood and lysate samples stored at 4 degrees C and -18 degrees C. In lysate samples kept frozen the GSH-Px activity remained stable over a period of 60 days while in plasma samples the activity had fluctuations. The GSH-Px activity in plasma was 12.2 +/- 11.7 nkat/ml, in whole blood 386 +/- 117 nkat/ml. The coefficient of correlation between plasma and whole blood GSH-Px activity was r = 0.27 (p less than 0.01), and between plasma and whole blood related to the hematocrit and to the hemoglobin content of the blood samples 0.38 and 0.35 respectively (both p less than 0.001). Between whole blood and whole blood related to hematocrit and hemoglobin content, r was 0.8 and 0.89 respectively (p less than 0.001).