Spermidine (SPD) and spermine (SPM) produced in anaesthetized dogs significant cardiovascular changes at higher doses than other transmitters, i.e. l-noradrenaline, l-adrenaline, histamine, acetylcholine, which produce cardiovascular effects at doses of 0.01-0.05 micrograms/kg given intravenously. SPD was shown to be more active than SPM. The hypotensive response observed after i.v. injection is due to histamine release. The hypotensive and bradycardic effects observed after microinjection of SPD into III cerebral ventricle or into the vertebral artery and of SPM into the vertebral artery are due to an increase in parasympathetic output. Spermidine increased and spermine decreased the baroreceptor reactivity. SPD and SPM did not change the vascular beta- and alpha-adrenergic, cholinergic and histaminergic receptor reactivity.