Sinus arrest induced by trivial nasal stimulation during alfentanil-nitrous oxide anaesthesia. A case is reported of bradycardia and sinus arrest induced by insertion of a nasal temperature probe. Other possible causes of bradycardia and sinus arrest under anaesthesia are reviewed briefly. Evidence for the neurological basis of a nasocardiac reflex, similar to the oculocardiac reflex, is presented. A minor, trivial stimulus may elicit this reflex. 