The properties of Ca2+ channels in strips and single muscle cells of longitudinal muscle of estrogen-dominated rat myometrium were studied under the effects of elevation of K+ concentration, the partial channel agonist Bay K 8644, and nitrendipine. In isolated strips in 0.5 mM Ca2+, Bay K 8644 (pD2 = 7.8–8.0) lowered the threshold for and enhanced the contractions in response to an elevation of K+ concentration, including the maximum response to K+ elevation alone. Bay K 8644 alone in concentrations up through 10−6 M did not initiate contractions in 0.5 mM Ca2+ solutions. At higher concentrations (10−5 M), Bay K 8644 behaved as an antagonist to contractions induced by elevation of K+. In isolated cells 10−7 M Bay K 8644 enhanced the shortenings to elevated K+ and lowered the threshold K+ concentration required. Also no significant contraction occurred with 10−7 M Bay K 8644 at normal K+ concentration. In contrast with its effect in isolated strips, no significant increase in maximum shortening (to 60 mM K+) was observed, possibly because cells without a mechanical load were maximally shortened by K+ alone. From these studies, we conclude that Ca2+ channels of isolated strips and cells of rat myometrium behave similarly and have similar properties to those of other smooth muscles in their interactions with elevation of K+, nitrendipine, and Bay K 8644.