Mouse uterine horns from 4 states (estrogen-primed and early-, mid-, and late-pregnancy) were used to study the effect of endothelin-1 (ET) vs carboprost (Car) and oxytocin (Oxy). In K(+)-Krebs (KCl 40 mmol.L-1) solution, ET (1-300 nmol.L-1), Car (0.002-20 mumol.L-1), and Oxy (0.6-60 nmol.L-1) evoked concentration-dependent increases in tension of the uterine horns from 4 different states. Emax for ET were 1.12 +/- 0.26, 1.27 +/- 0.18, and 1.49 +/- 0.13 g in early-, mid-, and late-pregnancies, respectively. Emax for Car in mid- was twice that in late-pregnancy, whereas Emax for Oxy in late- was thrice that in mid-pregnancy. EC50 for ET were 9.6, 5.8, and 6.3 nmol.L-1 in early-, mid-, and late-pregnancies, respectively, and were only 2% to 7% of that for Car and 3-15 times of that for Oxy in various gravid stages. The results suggest that the contractile activity of pregnant mouse uterus to ET is more potent than that of Car while slightly weaker than that of Oxy.