Objective: The aim of this study was to determine the effect of a single swimming exercise till exhaustion, in water on the morphological and cytological blood parameters in laboratory rats. Methods: The experiment involved 60 male rats from the Wistar strain, aged 8 weeks and 36 weeks. The animals were separated into 6 groups: 3 groups (I, II, III) of 8-week old animals, and 3 groups (IV, V, VI) of 36-week old animals. The rats in the study groups i.e. II, III, V, and VI were made to swim till exhaustion in water at 4ºC and at 25°C. After the experiment blood samples were collected from each rat and hematological and cytological determinations were completed for each sample. Results: Compared with the control group, in the rats from group II (8-week old, swimming in water at 4ºC) a significant increase was noted in the RBC count, Hb, Ht, and MCHC, and MCH indices, whereas in the rats from group V (36-week old, swimming in water at 4ºC) increases were noted in the MCHC and MCH indices (again when compared with the control group). In the rats from group III (8-week old, swimming in water at 25ºC), compared with the control group, significant increases were noted in the Hb and Ht parameters, as well as in the MCHC, and MCH indices, whereas in the group VI rats (36-week old swimming at 25ºC) – the increase was noted in the MCHC index. No alterations were found in the reticulocyte count, and in poikilocytes during exercise in water at low temperatures in either the 8-week or 36-week old rats. Conclusions: The increases in the majority of blood parameters, testifies to the dehydration of the body during physical exercise which in turn affects the microcirculation function. The lack of alterations in the reticulocyte count in the peripheral blood may, in turn, testify to the ineffectiveness of low temperature during exercise upon the function of the bone marrow.