Caracasanamide, one of the hypotensive agents isolated from Verbesina caracasana, is a mixture of (Z)-1a and (E)-1b forms of 1-[(3,4-dimethoxycinnamoyl)amino]-4- [(3-methyl-2-butenyl)-guanidino]butane. The structure of (E)-caracasanamide (1b) was confirmed by high-yielding synthesis starting from N,N'-bis(tert-butoxycarbonyl)-S-methylisothiourea. The water-soluble Z-form of 1a, assayed by i.v. route in anesthetized rats at doses ranging from 50 to 1600 micrograms/kg body weight, was found to decrease blood pressure, to increase cardiac inotropism, respiratory frequency, and tidal volume, and to induce a very slight and not significant tachycardia. Higher doses determined respiratory depression and, in some cases, consequent cardiac arrest. The compound was shown to affect cardiovascular function by acting at the vascular level in inducing arterial vasodilation, by determining sympathetic hypotone through central neurogenic mechanisms, and by interacting with the cardiac beta 1-adrenoreceptors. The respiratory effects were independent of the cardiovascular ones. In lowering blood pressure, the compound was more potent than guanethidine and not less potent than reserpine and papaverine. (Z)-Caracasanamide may therefore be useful in the treatment of arterial hypertension of moderate degree.