The aim of this study was to examine how increased body temperature affected the dimension of erythrocytes in the study group of rats. The total sample of 38 rats was divided into three experimental groups, depending on the exposure to water of the temperatures of 37°C, 41°C and 44°C. The group of rats exposed to the temperatures of 41°C-G41 and 44°C-G44 was divided into two subgroups: a subgroup on which antemortem analysis was performed after exposure to the temperature of 41°C and 44°C for

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... minutes (G41-AM=7; G44-AM=8) and a subgroup on which postmortem analysis was performed after death had occurred (G41-PM=8; G44-PM=8). Seven days before exposure to the temperature, blood sampling was performed in all three study groups. After exposure to the temperature for 20 minutes, blood sampling was performed in control group KG37 and subgroups G41-AM and G44-AM. After exposure to the temperatures of 41°C and 44°C in postmortem subgroups, sampling was performed after death (subgroups G41-PM and G44-PM). Morphometric analysis by the Giemsa method involved measuring of the diameter (μm) of the examined erythrocytes. The mean value of erythrocyte changes in the examined groups of rats was: KG37=6.57 ± 0.45; G41-AM=6.90 ± 0.22; G41-PM=6.79 ± 0.42; G44-AM=6.68 ± 0.43 and G44-PM=6.32 ± 0.37. Results of t-test show that there is a statistically significant difference between the erythrocyte diameters of subgroup G41-PM and G44-PM (p=0.049). There are significant changes in the diameter of erythrocytes, where the value of temperature as well as the exposure time significantly affect the diameter of erythrocytes.