The change pattern of SP and NO in the portal vein during the RAIR. 
To investigate the change pattern of substance P (SP) and nitric oxide (NO) in the portal vein during the recto-anal inhibitory reflex (RAIR), and its physiological significance; the influence of external splanchnic nerve (ESN) of rectum and anus to the RAIR.  Seventy-six rats divided into five groups according to the distance of Foley's tube in the rectum and whether or not to cut off ESN supply to the rectum and anal canal, to measure the values of SP and NO in the portal vein during the RAIR.  The stimulus in rectum can cause change of SP and NO in portal vein.  The greatest increase of SP is at the 6-cm group.  The 6-cm group with total ESN supply had significant difference compared with the 4-cm group before and after the ESN supply and control group were cut (P<0.01).  After cutting-off ESN, the increase of SP in the portal vein reduced significantly when compared with the normal ESN supply at the 6-cm group (P<0.05).  The greatest change of NO is at the 4-cm group with total ESN.  There were significant differences among the 4- and 6-cm groups and control group.  After cutting off ESN, the increase of NO was lower than with the intact ESN.  There were still differences between the 4- and 6-cm groups and control group(P<0.05).  The stimulations at different points of the rectum cause different SP and NO change in the portal vein.  This may be the explanation why the stimulation on the different points on the rectum induces different change pattern of RAIR from the neurotransmitters point.  The ESN supplies of the rectum and anal canal have an influence on the change of SP and NO in the portal vein during RAIR.