The acute potentiating effect of ouabain on norepinephrine (NE) induced contractions of isolated rabbit aorta was investigated. Ouabain, at concentrations of 3 × 10–7 to 10–5 mol/l potentiated the vasoconstrictor effect of NE as demonstrated by a shift to the left of the NE concentration-response curve in a parallel manner and a concentration-dependent increase in the EC50 ratio. A significant increase in maximal contractility was observed by ouabain at 3 X 10–6 and 10–5 mol/l. Removal of the endothelium did not alter the effect of 10–5 mol/l ouabain. When the NE concentration-response curves were again determined after ouabain had been washed out, it was found that the ouabain-induced increase in EC50 ratios returned to control values, while the increase in maximal contractility did not. The slopes of the NE concentration-response curves were not significantly different in the presence of ouabain than control values, but were higher when obtained after ouabain had been washed out.