1. The presence and partial characterization of immunoreactive atrial natriuretic peptide (ir-ANP) in the bulbus arteriosus were investigated using high performance liquid chromatography (HPLC) and radioimmunoassay (RIA) in several freshwater teleostean fish (catfish, silver crusian carp, snakehead, and rice eel). 2. The level of ir-ANP in the bulbus arteriosus was comparable with that in the atria at about 5-15 pg/mg, 6-13 pg/mg wet wt, respectively, but was 2-4-fold higher than that in the ventricles in all species observed. 3. The HPLC profile of ir-ANP showed two main peaks corresponding to low and high mol. wt rat ANPs. 4. Our results provide evidence for the presence of ir-ANP with high as well as low mol. wts in the bulbus arteriosus of freshwater teleostean fish.