The purpose of this investigation on human umbilical arteries was: (1) to analyze the effects of magnesium (Mg2+) in the prevention of contractions induced by 5-hydroxytryptamine (5-HT), KCl, Bay K 8644 and BaCl2; (2) to determine whether the presence of Mg2+ could modulate the response to caffeine in Ca2+-free medium at 25 and 37 degreesC, and (3) to examine the influence of Mg2+ and temperature in the release of intracellular calcium induced by 5-HT in Ca2+-free medium. Human umbilical arteries were dissected and mounted in organ baths for isometric measurement of force. Our results showed: (1) Mg2+ is a potent inhibitor of smooth muscle contraction of human umbilical arteries caused by 5-HT, KCl, Bay K 8644 and BaCl2, and that this effect might be due to the inhibition of the transmembrane calcium influx. (2) The relaxant effect of Mg2+ did not seem to be mediated by a blockade of the release of calcium from an intracellular store. (3) Moreover, our results suggest an effect of temperature independent from the action of Mg2+ on 5-HT-induced contraction.