The paper examines the influence of acute and chronic stress on the relationship between systolic blood pressure (SBP) and pulse interval (PI) recorded from laboratory rats with different genetic predispositions for the development of the hypertensive disease. Transfer entropy (TE) was used to examine the direction of information flow between SBP and PI, spontaneous baroreflex sensitivity (BRS) was used to evaluate the ability of adaptation of PI time series to changes in SBP, and the cross-approximate entropy (XApEn) to quantify the SBP-PI synchronization. The effects of the time series length on TE estimation was also investigated in an artificial environment for the time series without a strong causal relation. The consistency of the TE estimation was achieved only for extremely long time series. The results showed that chronic stress influence on the increase in information transmission between SBP and PI (TE) while changes of (BRS) and XApEn were not noticed.