As humic acid (HA) substances have antiphlogistic, adsorptive, antitoxic and antimicrobial properties, we studied the possible effects of Farmagulator((R)), an organic HA preparation, on the rat performance, nutrient retention, ileal histomorphology and hydroxyproline (HP) content of the ileum in two experiments. In experiment 1, 48 male Wistar albino rats were allotted to three dietary treatments. Each was randomly assigned to four cages, each with four rats. The treatments consisted of a control diet (C) with no addition of Farmagulator((R)) Dry or Liquid, a treatment with addition of 2.5 g/kg Farmagulator ((R)) Dry (FDry) and a control diet containing no FDry, but the rats had 3.5 ml/l Farmagulator((R)) Liquid in drinking water (FLiquid). The experiment lasted for 20 days. Changes in live weight were recorded at days 10 and 20 of the experiment. At the end of 20 days, all rats were killed to collect samples of intestinal tissues for the measurements of histological parameters. In experiment 2, 30 rats weaned at 21 days of age were divided into three groups, each with 10 rats, and individually caged in metabolism cages for 10 days. The above three treatments were randomly assigned to rats for 10 days to record body weight and feed intake. During the last 5 days, faecal outputs were collected to determine the dry matter and nitrogen retention. In experiment 1, FDry and FLiquid rats significantly (p<0.05) gained more live weight than the control rats. Improved weight gain with HA preparations was found to be highly associated with a high epithelial surface area as there were significantly (p<0.05) longer villi heights and crypt depths and increased HP contents of ileum in the HA treated rats compared with the control rats. Although the increased weight gain in FLiquid rats did not significantly (p>0.05) differ from the control rats in experiment 2 in contrast to the result in experiment 1, the FDry rats significantly (p < 0.05) gained more weight than the control rats. This was primarily found to be associated with significantly (p<0.05) increased feed intake and nitrogen retention in FDry rats compared with the control rats. It can be concluded that HA preparations, especially FDry, caused increased weight gain in rats as overall of two experiments. The improved weight gain only by FDry preparation was associated with increased ileal epithelial mass, increased feed intake, improved feed : gain ratio and increased nitrogen retention in rats.