1 The rectum isolated from the fowl was perfused with Tyrode solution via the caudal mesenteric artery. Noradrenaline and adrenaline were biologically or fluorimetrically assayed in perfusates collected before and during stimulation of Remak's nerve or of the periarterial nerves.2 Perfusates collected during nerve stimulation relaxed the chick rectum and rat stomach strips which served as assay tissues. This effect was attributed to the action of noradrenaline or adrenaline released from adrenergic nerve endings which appeared in the perfusates.3 Perfusates obtained during stimulation (30 Hz for 60 s) of Remak's nerve contained both noradrenaline and adrenaline when measured fluorimetrically. The mean output per stimulus train was 0.8 +/- 0.2 ng/g wet wt. tissue for noradrenaline and 1.7 +/- 0.2 ng/g wet wt. tissue for adrenaline (n = 7). Perfusates obtained during stimulation (30 Hz for 60 s) of the periarterial nerves contained noradrenaline in a concentration of 1.6 +/- 0.3 ng/g wet wt. tissue per stimulus train, but almost no adrenaline (n = 7).4 Neither stimulation of Remak's nerve nor the periarterial nerves liberated catecholamines when the rectum was perfused with Tyrode solution containing low Ca(2+) (0.1 mM) and high Mg(2+) (10 mM).5 Infusion of high potassium solution (50 mM) increased markedly the output of noradrenaline and adrenaline.6 Adrenaline as well as noradrenaline may function as the adrenergic neurotransmitter in the rectum of the fowl.