The contractile response of arterial smooth muscles induced by agonists as noradrenaline or histamine in Ca-free solution consists of two phases: an initial phasic component, which is transient and accompanied by a release of cellular Ca, and a small tonic component, which persists as long as the agonist is present. A second admission of the agonist without reexposure to Ca elicits only the tonic component. This tonic contraction differs in several respects from the phasic response obtained in Ca-free solution: it is independent of the duration of exposure to the Ca-free solution, it can be elicited many times without reexposure to Ca, and it is not accompanied by a measurable release of Ca from the cells. During superfusion with Ca-free solution, a tonic contraction is also induced by fluoride ions at concentrations exceeding 4 mM. The amplitude of this contraction is maximal at about 12 mM. Increasing the fluoride concentration shortens the delay between the addition of the F-- and the onset of the contraction. As is the case for the tonic noradrenaline-response, the F--induced contractions can be elicited many times without reexposure to Ca. The tonic contractions evoked by noradrenaline or histamine and by fluoride ions are additive. Both contractions are reversibly inhibited by caffeine, theophylline, Na-nitroprusside, papaverine and by nitroglycerine. The possibility that these tonic contractions are not accompanied by an increase of the cytoplasmic Ca2+ concentration is discussed.