This study examined the role of the renin-angiotensin and vasopressin systems on systolic blood pressure (SBP) variability following subarachnoid haemorrhage (SAH) in conscious rats. Animals received no treatment, the angiotensin II AT1 receptor antagonist, losartan, or the vascular vasopressin receptor antagonist, AVPX. SAH resulted in a transient sympathetic activation as estimated from the increase in the mid-frequency oscillations of SBP (3.2 +/- 0.8 mm Hg2, 3 hours after the injury vs. 1.3 +/- 0.3 mm Hg2 in control conditions, p < 0.01). On the second and fourth day following SAH, a marked elevation in the low-frequency component of SBP was observed (7.1 +/- 1.0 mm Hg2 on day 2 vs. 2.6 +/- 0.3 mm Hg2 in control conditions, p < 0.001 and 6.3 +/- 1.1 mm Hg2 on day 4 vs. 2.6 +/- 0.3 mm Hg2 in control conditions, p < 0.01). Pre-treatment with losartan prevented the acute rise in the mid-frequency oscillations in SBP and partially reduced the low-frequency component observed at 2 and 4 days. Administration of AVPX on the second and fourth day following SAH normalised the elevated low-frequency oscillations in SBP. This study indicates that the modifications in SBP variability observed in the early and delayed stage after subarachnoid haemorrhage involve angiotensin II. Vasopressin seems to be implicated in the delayed development of low-frequency fluctuations of SBP.