SUMMARY: When isolated eel gills were perfused under conditions resembling, as closely as possible, those found in vivo and under constant pressure, neurohypophysial hormones decreased and catecholamines increased the rate of flow of perfusate, the latter response being mediated by β-adrenergic receptors. When the Ringer solution was not filtered before use, flow rates rapidly declined and 10−5 m-adrenaline or noradrenaline was required to maintain constant flow. Under these conditions, 10−14 m-isotocin or 10−13 m-arginine vasotocin (AVT) produced vasoconstriction. When the Ringer solution was filtered through a 0·22 μm Millipore filter before use, constant high flow rates could be maintained in the absence of catecholamines. Noradrenaline increased the flow rate at concentrations of down to 10−9 mol/l, adrenaline being slightly less active when filtered Ringer solution was used. However, the sensitivity to neurohypophysial hormones was considerably reduced, 10−11 to 10−10 m-isotocin and 5 × 10−11 to 5 × 10−10 m-AVT being the lowest concentrations producing vasoconstriction. No difference in sensitivity to neurohypophysial hormones or catecholamines was observed between gills from seawater adapted or freshwater adapted gills. The results are discussed in relation to the alternative pathways of blood flow through the gills.