Noradrenaline induced vasodilation in isolated perfused eel or trout gills was inhibited by propranolol, aβ‐adrenergic blocking agent. The noradrenaline effect was much reduced in gills of fish which had been kept in 1 mg/l solutions of the detergent LAS (linear alkylate sulphonate; 12C chain length). LAS produced concentration‐dependent vasodilation, which was inhibited by propranolol, in eel and trout gills. Noradrenaline produced additional vasodilation in eel and trout gills which had been dilated by 2.10−8mLAS but had no effect on trout gills perfused with 2.10−7mLAS. Gills from trout which had been kept in 0.1 mg l‐1LAS responded to noradrenaline normally but the propranolol inhibition appeared to be p