The effect of a recently developed oxytocin antagonist dTVT, i.e. deamino-[2-D-tyrosine(OEt)-4-threonine-8-ornithine] oxytocin on uterine contraction of pregnant rats was studied in vitro. The following results were obtained. 1. dTVT treatment did not affect spontaneous PGE2- or PGF2 alpha-stimulated contraction, while it slightly suppressed PGE1 analogue (Gemeprost)-stimulated contraction of the uterus. 2. Following treatment with dTVT (5-50 micrograms/ml), oxytocin-stimulated uterine contraction was gradually and slowly suppressed, resulting in an attenuation curve. Ritodrine treatment, on the other hand, rapidly suppressed spontaneous uterine contraction as well as contraction stimulated by various oxytocics. Suppression of oxytocin-stimulated uterine contraction by dTVT took much longer (14.8 +/- 1.1 min) to take effect than that by ritodrine (less than 1 min).