1. Increase of Ca2+ concentration in the bath solution diminishes the contractile response of isolated rabbit aorta rings to alpha 1-adrenoceptor agonists and KCl. 2. In intact preparations the contractions of methoxamine and phenylephrine were maximal when a 0.3- to 0.6-mM Ca2+ bath solution was used, and those of KCl were maximal with a 2.5-mM Ca2+ concentration. 3. The contractions of methoxamine and phenylephrine also were decreased by increasing the Ca2+ concentration above 1.25 mM in disrupted endothelium preparations and in those incubated in indomethacin (10(-5) M), N omega-nitro-L-arginine methyl ester (10(-4) M), or methylene blue (10(-6) M). 4. High organ bath Ca2+ concentrations also caused a decrease in KCl contractions using endothelium-denuded and the treated preparations, the responses being similar with 1.25 mM and 2.5-mM Ca2+ in the methylene blue-treated preparations, whereas they were greater with 1.25 mM Ca2+ in the others.