Radiotelemetred male Wistar outbrad rats and Borderline Hypertensive rats (BHR) were exposed to acute and chronic environmental stress. Approximate entropy (ApEn) approach is applied in order to investigate the pulse interval (PI) response to two different types of environmental stress: shaker and restrain stress. The performance of ApEn method was evaluated from the parameter selection point of view. The purpose of the study is to quantify the complexity of response to stress and period of recovery after the stress in order to gain an insight in consequences of chronic stress exposure.