Panel A: Myogenic tone was determined in nonpregnant uterine arteries after pretreatment with 17β-estradiol (E2β, 0.3 nM) plus progesterone (P4, 100 nM) or vehicle control for 48 h at given pressures. Panel B: Myogenic tone was determined in pregnant uterine arteries after pretreatment with ICI 182,780 (10 μM ) plus RU 486 (1 μM) or vehicle control for 48 h in the presence of E2β (0.3 nM) plus P4 (100 nM) at given pressures. Myogenic tone was determined in the presence of 100 μM L-NNA. Data are means ± SEM of 4-5 animals. * P < 0.05, significant difference between control and treatment groups, as determined by repeated-measures, two-way ANOVA.