The effects of high potassium, carbachol and oxytocin on 40Ca-45Ca exchange in the estradiol-treated rat uterine longitudinal muscle were compared with those in the ovariectomized one using the modified "Lanthanum Method." The treatment of estrogenized or ovariectomized myometrium with 60 mM K+ significantly increased the 45Ca space at 5-, 10-, 30- and 60-min incubation times. The increase in the 45Ca by high-K+ in the estrogenized myometrium was much greater than that in the ovariectomized one. Carbachol (2 X 10(-6) M) had little influence on the 45Ca space at all incubation periods with the exception of the 10-min one in both uteri. The higher concentration of carbachol (2 X 10(-4) M) in the estrogenized myometrium produced a marked increase in the 45Ca space after the 10-min incubation, while in the ovariectomized uterus, a slight increase in the 45Ca space was observed at the 10- and 30-min incubation times. The increase in the 45Ca space by 2 X 10(-4) M carbachol in the estrogenized myometrium was greater than that in the ovariectomized one. Oxytocin (10(-4) and 10(-3) units/ml) increased the 45Ca space at and after the 10-min incubation times in the estrogenized uterus, while in the ovariectomized uterus, oxytocin (10(-3) and 10(-2) units/ml) showed little influence on the increase in the 45Ca space. The increase in 45Ca space by carbachol and oxytocin was inhibited by papaverine. These results suggest that differences in dependence upon Ca2+ in the medium may exist in contractions induced by high-K+, carbachol and oxytocin in the uterine smooth muscle and that the contractions in the estradiol-treated myometrium may be more dependent upon extracellular Ca2+ than those in the ovariectomized one.