1. Haematological parameters and cytomorphological picture of circulating blood of the grass snake Natrix natrix natrix L. were studied. Mean annual values of the haemoglobin concentration, haematocrit level, red cell counts, erythrocytes indices, erythrocyte sedimentation rate, thrombocyte and leucocyte counts, per cent composition of white blood cells and size of morphotic elements were determined for a population sample (N = 154) of the species, considering the sex of snakes. 2. Parameters of red blood cell system (RCC, Hb, Hct) were statistically significantly higher in males, remaining indices showed no statistically significant differences. 3. A strict positive correlation was found between RCC, Hb and Hct in the studied sample. 4. Cytomorphology of blood is presented in microphotographs. 5. In the discussion haematological data for the grass snake and other snakes are compared.