The haemodynamic alterations induced by the central and peripheral administration of the armed spider (Phoneutria nigriventer) venom (PNV) were investigated in anaesthetised rabbits. The intracerebroventricular injection of increasing doses of PNV (30 and 100 microg/kg) elicited a biphasic cardiovascular response characterised by a brief hypotension (1-3 min) followed by a marked and sustained (more than 30 min) increase in mean arterial pressure (61 +/- 5 and 61 +/- 10%, respectively) and in systemic vascular resistance (135 +/- 21 and 161 +/- 37%) accompanied by mild increases in cardiac contractility. Systemic alterations such as salivation and muscular fasciculation were also observed. At the opposite, the dose of 100 microg/kg of PNV injected intravenously produced only a hypotensive effect (29 +/- 4% decrease in mean arterial pressure) and a decrease in vascular resistance (38 +/- 5%). Nevertheless, a much higher dose of PNV (1 mg/kg) injected intravenously produced a hypertensive response analogous to the one observed upon central administration. The central hypertensive response induced by PNV was not affected by preteating the animals with selective antagonists of receptors of different neurotransmitters or endogenous mediators such as: acethylcoline muscarinic, bradykinin B2, angiotensin II AT1 receptors and also antagonists of the excitatory amino acid receptors of the central nervous system. Nevertheless, the intravenous pretreatment with the selective alpha1-adrenergic receptor antagonist prazosin significantly blunted the excitatory cardiovascular response evoked by the central injection of PNV. It is concluded that PNV can induce central as well as peripheral haemodynamic effects. The central component seems to be mediated by the activation of cardiovascular centres which in turn lead to an increase in the sympathetic outflow to the periphery, whereas the peripheral component can be accounted for either by direct activation of the vascular alpha1-adrenergic receptors or by catecholamine release from the sympathetic nerve endings.