The fermentation of pectin by colonic bacteria produces short-chain fatty acids (SCFA) which are then absorbed by the host. The purpose of this study was to determine whether pectin, added to a chemically defined diet, would increase hepatic lipogenesis and whether this effect is mediated by intestinal bacteria. Eighteen Sprague-Dawley rats underwent placement of a feeding gastrostomy and a swivel apparatus. Postoperatively, rats were randomly assigned to one of three groups: 1) No Pectin received a fat-free chemically defined diet, 2) Pectin received the same diet with the addition of 1% (w/v) pectin, and 3) Neomycin received the same diet with 1% w/v pectin and neomycin (80 mg/kg of body weight daily). On the 5th postoperative d, all diets included 12.5% (v/v) deuterium as D2O. After the infusion of the labeled diets for 24 hr, the content and deuterium enrichment of liver palmitate, stearate and oleate were measured and the production rates calculated. The liver content and production rates of these fatty acids were higher in Pectin animals than in either the No Pectin or Neomycin animals. Since the effect of pectin on hepatic lipogenesis was reduced by the concomitant administration of the intestinal antibiotic neomycin, it appears that this effect depends on the bacterial fermentation of pectin. It is postulated that the SCFA produced during pectin fermentation promote lipogenesis via a direct stimulatory effect, in addition to being carbon donors.