BACKGROUND: Serotonin is found in the blood of the umbilical cord at birth in concentrations sufficiently high to affect vascular tone. Serotonin has been suggested to be involved in the pathogenesis of preeclampsia. Magnesium sulfate (MgSO4) is used to treat convulsions and hypertension in patients with preeclamptic toxemia. Bupivacaine is used in the epidural anesthesia for a cesarean section. The effects of magnesium and bupivacaine on serotonin-induced vasocontraction in a human umbilical artery was investigated. METHODS: Experiments were performed on 52 human umbilical arteries. The rings were suspended in an organ bath to record isometric mechanical activity. The concentration-contraction responses to bupivacaine, magnesium and serotonin were measured respectively. Vessels were pretreated with bupivacaine (10(-5) M) or magnesium (2 mM or 6 mM), and then serotonin (10(-9) M - 10(-6) M) was added cumulatively. Data analysis was assessed by an unpaired t test, one-way ANOVA and a Kruskal-Wallis test. RESULTS: Bupivacaine induced a contraction of umbilical arterial rings, and showed a maximal contraction (51.8 +/- 6.1%) at a concentration of 43nM. Magnesium induced relaxation of the umbilical artery in a concentration dependent manner. Pretreatment with bupivacaine (10(-5) M) potentiated significantly the concentration response to serotonin (P < 0.05). Pretreatment with MgSO4 (2 mM or 6 mM) significantly suppressed the contractile response to serotonin (P < 0.05). CONCLUSIONS: Bupivacaine, magnesium and serotonin are vasoactive on human umbilical arteries. Magnesium exerts a strong relaxant effect on serotonin induced vasocontraction in the human umbilical artery. Potentiation of serotonin induced vasoconstriction by bupivacaine may play a significant role in the reduction of umbilicoplacental blood flow.