The influence of Ca2+ on the antioxytocin actions of papaverine, deoxycholate and 1,1-diphenyl-3-piperidinobutanol hydrochloride (Aspaminol) was investigated in the ovariectomized and the estradiol-treated rat uteri, and Ca exchangeability and Ca content in both uteri were compared. No qualitative difference in the responses to deoxycholate and Aspaminol at normal and higher Ca2+ concentrations in the estradiol-treated rat uterus was seen as compared to those obtained from the ovariectomized one. The antioxytocin action of papaverine in the estrogenized uterus was antagonized by excess external Ca2+, but not in the ovariectomized one. There were no significant differences either in the total Ca content or the non-displaceable Ca by La between the ovariectomized and the estrogenized rat uterine longitudinal muscle layers, while Ca exchangeability in the estrogenized was much greater than that in the ovariectomized one. These results suggest that the antagonism of papaverine against exogenous Ca2+ may be related to the more increased Ca exchangeability in the estradiol-treated rat myometrium.