Lanthanum (La3+) (2 mM).initially increased efflux of 45Ca from rings of rabbit aorta, but also caused an increased retention of 45Ca after prolonged washing of the tissue in La3+-containing solution for 60 min. La3+ was also found to inhibit the uptake of 45Ca into this preparation. Exposure to high potassium for 60 min during uptake of 45Ca increased the amount of tracer retained after the La3+ wash, but the extra amount retained was in excess of that required to activate contraction, and much more than the extra retention after exposure to high potassium for 5 min sufficient to produce maximal tension. Exposure to noradrenaline during 45Ca uptake produced a smaller extra retention of isotope following La3+ washing. In the presence of high potassium, exposure to noradrenaline had a greater effect on the extra retention of 45Ca.The potassium-induced contracture of rabbit aortic rings was prevented and/or relaxed by La3+ (2 mM); and while the sustained response to noradrenaline was similarly affected, the initial phasic response to this agent could still be elicited in the presence of La3+.These results suggest that La3+ can displace Ca2+ from extracellular sites and inhibit influx of extracellular Ca2+ and part of the efflux of intracellular Ca2+. The potassium-induced contractures and the sustained response to noradrenaline involve transmembrane Ca2+ movements, while the phasic response to noradrenaline does not. However, results of 45Ca uptake studies cannot be interpreted as showing that La3+ seals the membrane to Ca2+, or that extra Ca2+ retained by La3+ is related to contraction.