Extracts of urophysis of crucian carp (Carussius auratus L.)induced rhythmic contractions of the intestines of mandarin fish (Siniperca chuatsiB.)in vitro. The contractfion effect of the extracts showed a high degree of sensitivityand good reproducibility. The experiments was found to be dose-dependent. Repro-ducible responses were obtained with as little as 0.05 of a urophysis/ml in fluid bath.It was suggested that the isolated mandarin fish intestines could be used to substitutefor trout rectum in order to provide the bioassay method for the estimation of urote-nsin Ⅱ. Acetylcholine Was used for a laboratory reference standard.