1. This study examines the effects of methanolic extract (ME) and its main constituent, sorocein A, isolated from the roots of Sorocea bonplandii on agonist-induced contractions in the rat uterus (RU) and in the guinea pig ileum (GPI) in vitro. 2. ME (25-100 micrograms/ml), added to RU for 20 min, caused a graded and parallel shift to the right of bradykinin (BK)-mediated contractions with an apparent pA2 value (-log g/ml) of 5.0 ME caused a rightward shift of the acetylcholine (ACh) and oxytocin-induced contractions associated with a marked depression of their maximal responses. 3. In GPI, ME produced a non-competitive antagonism against BK-induced contraction, while responses to ACh and histamine were shifted to the right in a graded fashion, yielding pA2 values (g/ml) of 5 in both cases. 4. The purified compound sorocein A (15-60 microM) caused a parallel and graded rightward displacement of BK and ACh concentration-response curves in RU with pA2 values (molar basis) of 4.9 and 5.2. 5. Sorocein A also dose-dependently shifted to the right ACh and histamine-mediated contractions in GPI, yielding pA2 values of 5.1 and 4.8, respectively. 6. However, sorocein A antagonized in a non-competitive manner BK-induced contraction in GPI, characterized by a graded displacement to the right of the dose-response curve, and progressive depression of the maximal contraction.