Background: Characterization of functional differences between lower oesophageal sphincter (LOS) clasp and sling muscles might aid the development of more specific pharmacological and surgical approaches for the treatment of motility disorders. Methods: Circular LOS strips from 25 adult pigs were studied in organ baths to compare the physiology of clasp and sling fibres. Results: Sling strips developed greater tone than clasp fibres (mean(s.e.m.) 7.59(0.89) versus 4.72(0.67) g; P = 0.017). LOS tone was more dependent on extracellular calcium in clasp strips and on the activity of cholinergic enteric motor neurones (EMNs) in sling strips. The amplitude of maximal relaxation caused by electrical field stimulation (EFS, 3Hz) of EMNs was greater in clasp strips (mean(s.e.m.) 74.5(2.3) versus 58.1(2.2) per cent of tone; P < 0.001). EFS-induced relaxation was reduced in clasp fibres and fully blocked in sling fibres by nitrergic blockade with 10 μmol/l 1H-[1,2,4]oxadiazole-[4,3- α]quinoxalin-1-one (ODQ). The amplitude of EFS cholinergic responses was significantly greater in sling fibres. In the clasp region, relaxation caused by stimulation of EMNs with 100 μmol/l nicotine was reduced by ODQ. In sling fibres, nicotine induced relaxation at rest and cholinergic contraction following ODQ. Conclusion: Clasp and sling fibres of the porcine LOS show marked intrinsic functional differences. This should be considered when developing more specific approaches to human LOS motility disorders. Copyright © 2007 British Journal of Surgery Society Ltd Published by John Wiley & Sons Ltd.