An osmoregulatory role for angiotensin II was investigated in the euryhaline European eel Anguilla anguilla L., plaice (Pleuronectes platessa) and dab (Limanda limanda). 1. Ile5 - and val5 -AII increased blood pressure in all species. Ile5 -AII produced a greater effect than val5 - AII, at equal concentrations, in eels, however in plaice and dab, this trend was reversed. Papaverine reduced blood pressure, followed by recovery to control in all cases. 2. Both AII sequences increased drinking rates except ile5 -AII in dab. ile5 -AII again proved more potent than val5-AII in eels and val5 - was greater than ile5- AII in flatfish. Papaverine increased drinking in all species. Captopril had no effect on eel or dab drinking rates, but reduced those of plaice. 3. 1.0nM and 10.0nM AII increased ANP release from isolated eel myocytes, except from FW atria. At 1.0nM, ventricular release was greater than atrial, however at 10.0nM, AII ANP secretion was similar in both types of myocyte. In all cases ANP release was greater from SW than FW myocytes. 4. Tissue/plasma ratios revealed greatest binding in SW eel liver. Tissue receptor specific binding was also greatest in FW eel liver membranes, however in SW tissues gill was slightly greater than liver. A FW eel liver membrane radioreceptor assay was developed. Binding was optimal at 22°C, 25mM calcium, protein concentration of 700ug, 125I-AII concentration of 25pM and an incubation period of 60 minutes. 6. Initial FW eel liver binding studies indicated two receptor classes with Kd=1. 5x10-11M and 2.46x10-10. However subsequent studies reveal Kd =3.31x10-8M in FW liver and Kd=1.09x10-7M in SW liver preparations. 7. 125I-ile5 -AII produced greater binding than 125I-val5 -AII in eel liver preparations. 125I-val5 -AII produced greater binding in flatfish membranes. Investigations of 125I-ile5 -AII displacement from FW eel liver membranes revealed peptide potencies in the following order, sar-AII > ile5-AII > 5-8AII > ile5-AI > val5-AI > ile4-AIII > val4-AIII > bradykinin > 1-4AII.