The effects of raubasine and tetrahydroalstonine (THA) on the vasopressor response to sympathetic nerve stimulation and to i.v. administration of noradrenaline were studied in the pithed rat to ascertain whether raubasine and THA would preferentially block pressor responses due to exogenous versus endogenous noradrenaline alpha-adrenergic receptor activation. Raubasine (0.5 to 4 mg/kg i.v.) was more effective in blocking the response due to sympathetic nerve stimulation than that due to i.v. noradrenaline. On the other hand THA (0.5 to 4 mg/kg i.v.) produced greater inhibition of the i.v. noradrenaline response than the sympathetic nerve stimulation response. THA (0.5, 1, 2 mg/kg i.v.) enhanced the nerve stimulation response, while at the 4 mg/kg dose the response was slightly reduced. This may be explained by a preferential block by THA at low doses, of presynaptic alpha 2-adrenoceptors. In pithed rat raubasine and THA showed dose-related blocking effects on the pressor response of phenylephrine and B-HT 933, respectively. This suggest that raubasine preferentially blocks alpha-1 and THA alpha 2-adrenoceptors. The results suggest that raubasine and THA preferentially block the pressor responses of post-synaptic alpha-adrenergic receptor activation due to endogenous and exogenous noradrenaline, respectively.