The effects of adenosine on the calcium contraction of high K+ (low Na+)-depolarized preparations, as well as on the uptake and release of 45Ca, were studied in the guinea-pig taenia coli. Adenosine (10(-4)-10(-3) M) inhibited the calcium contraction of high K+-depolarized preparations, an effect which was mimicked by ATP (10(-4)-10(-3) M), but not by the slowly degradable ATP analog, adenosine 5'-alpha, beta-methylene triphosphate. The high K+ -stimulated calcium influx measured by 3-6 min exposures to 45Ca was not changed by adenosine (10(-3) M). In high K+ solution, adenosine (10(-3) M) increased the fractional rate of efflux of 45Ca-labelled preparations. These results indicate that stimulation of calcium extrusion, but not inhibition of calcium influx, may take part in the mechanism of the relaxation induced by adenosine in the taenia.