1. The effects of the addition of Ca2+ on ouabain-resistant 22Na+ efflux from Na+-loaded strips of rabbit portal anterior mesenteric vein in Ca2+-free media have been studied. 2. Na+ efflux into Li+ media containing 5 mM-KCl is rapidly and transiently stimulated some 4- to 5-fold on the addition of Ca2+ (1.2 mM). No stimulation is observed if the Li+ medium is K+ free or if Na+ replaces Li+ ions. This Ca2+-activated Na+ efflux is not obligatorily coupled to Na+ influx. 3. The stimulation of Na+ efflux could also be triggered by the addition of 5 mM-K+ to a Ca2+-containing K+-free medium. The Ca2+-activated increase in Na+ efflux also occurred when K+ was the sole monovalent extracellular cation. Rb+ could substitute for the K+ requirement. Thus the Na+ efflux is not mediated by a system which has a specific requirement for counter-transport of Li+ or one in which Li+ but not K+ are counter-transported such as the familiar Na+-H+ exchange system. Acidification of the external medium reduced the Ca2+-stimulated Na+ efflux, in keeping with the conclusion that this efflux was not due to Na+-H+ exchange. 4. Progressive reduction of external [Ca2+] increased the time-lag to peak activation of Na+ efflux, suggesting that the effects of added Ca2+ were mediated by a rise in intracellular Ca2+. Under experimental conditions which did not result in activation of the Na+ efflux by the addition of extracellular Ca2+ alone (e.g. in Na+ media), addition of Ca2+ plus the Ca2+ ionophore, ionomycin, stimulated Na+ efflux. This further confirms that intracellular sites for Ca2+ are critical for the activation of Na+ efflux. In the absence of ionophore, in Na+ media, intracellular Ca2+ is not sufficiently increased when extracellular Ca2+ is added. A partial (40%) block of Ca2+-activated Na+ efflux by amiloride (2 X 10(-3) M) could also be overcome by the addition of ionomycin. 5. The lack of effect of a variety of inhibitors suggests that the Ca2+-stimulated Na+ efflux mechanism is not mediated via a Na+-K+-Cl- co-transport system or a Na+-H+ counter-transport system, or Na+-Ca2+ exchange. 6. The activation of Na+ efflux in smooth muscle by Ca2+ ions seems to involve Ca2+ entry partially via an extracellular Ca2+-intracellular Na+ exchange and also through other parallel pathway(s), followed by a rise in intracellular Ca2+ that activates Na+ efflux through a Ca2+-sensitive Na+ channel or other transport pathway.