Mercuric chloride and lead acetate caused contraction of rabbit aortic segments in vitro. Mercuric chloride was added to a Krebs Henseleit solution; lead acetate was added to a Tris-buffered medium to avoid any precipitation of lead. Cadmium (Cd++) acetate, in either medium, had no direct effect. The Hg++- and Pb++-induced contractions were significantly reduced when Ca++ was omitted from the physiological buffer or when pH decreased. The alkylated analogues of Hg++ and Pb++ salts had little direct contractile effect. However, segments exposed to these analogues lost their ability to respond to other agonists. This report provides evidence that Hg++ and Pb++ can directly induce active tension by action on processes which rely on extracellular Ca++.