1. 1. Progesterone concentration and haematological parameters at different stages of estrous cycle of the common vole have been investigated. 2. 2. During the estrous cycle progesterone concentration varied between 1.08 and 7.69 ng/ml of plasma; in the metaestrous period it was 7.69 ng/ml and 1.08 ng/ml in the proestrous period. 3. 3. The highest haemoglobin level, hematocrit value and erythrocyte number have been found in the proestrous period, and the lowest, in the estrous and metaestrous periods. 4. 4. The reverse changes were observed in the total and differential number of leukocytes. 5. 5. Statistically significant changes in all these parameters have been found.