A modification of an apparatus for undirect bloodless measurement of the arterial blood pressure (BP) in the tail of small laboratory animals (rats) is described. The device consists of a 3-channel ECG apparatus (C3a), connected with a blood pressure recorder(R67z/32) with an osciliscope (R17e) for visual checking of blood pressure. Criteria as well as possibility for measuring not only the systolic, but also the diastolic level of the blood pressure are indicated due to the great sensitivity of the device. It is pointed out that the coefficient of variation (3,58% for the systole and 5,95% for the diastole) is very low which confirms the reliability of the method. The results of a continuous (12 months) dynamic follow-up of the systolic and diastolic blood pressure of normotensive (NWR) and spontaneously hypertensive rats (SHR) are also reported. The authors conclude that the results could be used as normatyves and as indices for the range of the systolic and diastolic blood pressure of NWR and SHR aged 1 to 12 months, since no data about the diastolic blood pressure of rats are available so far in the literature.