1. The effect of different vasoactive agents on segments of human chorionic arteries and veins was analyzed. 2. The order of these agents to produce maximal contractile responses was: 5-hydroxytryptamine (5-HT) = histamine (H) = K+ greater than noradrenaline (NA) greater than or equal to phenylephrine (PHEN) greater than clonidine (CLON), and with regard to their potencies (EC50 values) was: 5-HT greater than or equal to NA greater than or equal to H greater than PHEN greater than CLON greater than K+. Dopamine and isoproterenol did not elicit any type of response. 3. The receptors involved on the contractions elicited by agonists were analyzed. 4. The results obtained suggest that: (1) 5-HT is the most potent vasoconstrictor agent of all those tested, whose effects appear to be mediated by 5-HT2- but not by alpha 1-adrenergic receptors; (2) chorionic vessels also possess H1-receptors; and (3) ketanserin has more affinity to block 5-HT2-receptors than to block alpha 1-adrenergic- and H1-receptors.