An investigation of rat jejunal and distal colonic electrolyte transport in vitro was undertaken using an Ussing chamber prepartion. Selective a2-adrenoceptor stimualtion in the jejunum was found to depress theo-phylline elevated anion secretion, as evidenced by decreases in short- circuit current (SCC). or i -Adrenoceptor stimulation, after a2 -adrenoceptor antagonism in the jejunum, evoked transient increases in basal anion secretion, as reflected by transient increases in basal SCC. The use of the neurotoxin tetrodotoxin indicated that this was a direct epithelial secretory effect. 5-hydroxytryptamine (5-HT) on the jejunum elicited transient increases in basal anion secretion, as demonstrated by transient increases in basal SCC. The use of tetrodotoxin, reserpine and a i -adrenoceptor antagonists, indicated that a major component of this epithelial secretory effect by 5-HT, was associated with activation of intramural nervous pathways of the sympathetic nervous system, ultimately stimulating a i -adrenoceptors. This might represent an important secretory mechanism by 5-HT in the jejunum. ^32-Adrenoceptor stimulation in the distal colon was found to decrease basal SCC, as evidenced by the metoprolol resistant effect of the selective j82- adrenoceptor agonist salbutamol, and lack of effect of the selective /9i-adrenoceptor agonist prenalterol. An investigation of rat distal colonic fluid and electrolyte transport in vivo was undertaken using an colonic loop technique. Although a basal colonic absorption of Na+ and Cl~, and a secretion of K+ were observed, these processes were not under tonic a-adrenergic regulation, as evidenced by the lack of effect of selective a-adrenoceptor antagonism. The secretory effects of prostaglandin-E2 were inhibited by a-adrenoceptor activation, whereas such stimulation did not evoke pro-absorptive responses upon basal transport, unlike noradrenaline.