Initial studies on the digestive hormone neurotensin (NT) showing that intestinal NT mRNA expression and blood levels were altered in rats fed chow containing bile acid (BA) and the BA chelator cholestyramine led us to investigate the role of NT in the enterohepatic circulation of BA. In fasted, anesthetized rats with common bile ducts cannulated for bile collection, intravenous NT infusion (10 pmol · kg¹ · min¹) enhanced BA output relative to control over 3 h in animals administered donor bile into the duodenum (30 µl/min). This suggested that the effect of NT was on the return of BA from the intestine to the liver, which is rate determining in the normal process. In rats prepared as described above and administered [³H]taurocholate ([³H]TC; 5 mM, 1 ml) duodenally, NT infusion (3-10 pmol · kg¹ · min¹) increased the [³H]TC recovery rate in bile approximately twofold, whereas sulfated CCK-8 (12-50 pmol · kg¹ · min¹) had no effect. To investigate the roles of endogenous NT and CCK, we administered [³H]TC into the rat duodenum or lower jejunum and tested the effect of the NT antagonist SR-48692 (2 nmol · kg¹ · min¹) or CCK-A antagonist lorglumide (100 nmol · kg¹ · min¹). SR-48692 reduced the [³H]TC recovery rate by 50% and 24% in the duodenum and jejunum, respectively, whereas lorglumide had no effect. These results suggest that NT or a similar peptide is an endogenous regulator of enterohepatic BA cycling, which acts by enhancing BA uptake in the intestine.