This paper describes the effects of vasopressin nonpeptide selective V1a (OPC-21268) and V2 (OPC-31260) antagonists on fast blood pressure (BP) oscillations in conscious non-haemorrhaged and haemorrhaged rats. Equidistant sampling at 20 Hz allowed direct spectral analysis of BP on 30 overlapping 2048 point-time series. In non-haemorrhaged rats, V1a antagonist (5 mg/kg; i.v) reduced BP and low-frequency (LF-BP) component while subsequent administration of V2 antagonist (1 mg/kg; i.v) reversed these changes and enhanced the very low-frequency (VLF-BP) component. In haemorrhaged rats (5-15 ml/kg/min) V2 antagonist pre-treatment enhanced the VLF-BP component during normotensive bleeding, while the V1a antagonist pre-treatment modified BP variability after hypotensive haemorrhage by enhancing the HF-SBP component. The results suggest that under normotensive conditions vasopressin by the stimulation of both V1a and V2 receptors buffers BP variability in the VLF-BP frequency domain. In addition, under hypotensive conditions vasopressin, by the stimulation of V1a receptors buffers the respiration-induced HF-BP oscillation.