Abstract 1. 1. In helical strips prepared from rat tail arteries, hydralazine preferentially depressed the slow phase of noradrenaline-induced contractions and the rapid phase of KCl-induced contractions. 2. 2. Its effect was lost in a calcium free medium and in strips depolarized with KCl and subsequently contracted with noradrenaline. 3. 3. The results suggest that hydralazine, at least at high concentrations, may interfere with the transmembrane movements of calcium ions in vascular smooth muscle.