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1. The contractile responses of rat vas deferens to noradrenaline and K+ are composed of phasic and tonic components both of which are dependent upon the concentration of extracellular Ca2+. 2. Lanthanum, La3+, and thulium ions, Tm3+, inhibited the noradrenaline and K+ induced responses, complete inhibition being obtained at approximately 10(-3) M-Ln3+. 3. La3+ and Tm3+ were equally effective in inhibiting noradrenaline and K+ responses. The phasic and tonic components of the noradrenaline response were equally sensitive to lanthanide cations, Ln3+, but the phasic component of the K+ response was more sensitive than the tonic component. 4. 170Tm binding did not show any saturable component over the concentration range in which inhibition of the pharmacological response was obtained. 5. It is suggested that the actions of Ln3+ in the rat vas deferens are mediated through some kind of membrane stabilization rather than via a specific Ca2+ binding site concerned with excitation-contraction coupling, the mechanism previously postulated for the Ln3+ action in guinea-pig ileal longitudinal muscle.