When oil is spilled into aquatic systems, chemical dispersants frequently are applied to enhance emulsification and biological availability. In this study, a mammalian model system was used to determine the effect of Bonnie Light Nigerian crude oil, weathered for 2 d with continuous spraying and recirculation, and a widely used dispersant, Corexit (Cx) 9527, on intestinal microbial metabolism and associated populations. To determine the subchronic dose, concentrated or diluted (1:2, 1:5, 1:10, 1:20) Cx9527 or oil was administered by gavage to Fischer 344 rats and the effect on body weight was determined. Next, rats were treated for 5 wk with oil, dispersant, or dispersant + oil. Body and tissue weights, urine mutagenicity, and the impact on the intestinal microflora and three microbial intestinal enzymes linked to bioactivation were determined in the small and large intestines and cecum. Two tested dispersants, Cx9527 and Cx9500, were toxic in vitro (1:1,000 dilution), and oil was not mutagenic in strains TA98 and TA100(+/-S9). None of the treated rats produced urine mutagens detected by TA98 or TA100. Undiluted dispersant was lethal to rats, and weight changes were observed depending on the dilution, whereas oil generally was not toxic. In the 5-wk study, body and tissue weights were unaffected at the doses administered. Small-intestinal levels of azoreductase (AR), beta-glucuronidase (BG), and nitroreductase (NR) were considerably lower than cecal and large-intestinal activities at the same time point. A temporal increase in AR activity was observed in control animals in the 3 tissues examined, and large-intestinal BG activity was elevated in 3-wk controls. No significant changes in cecal BG activity were observed. Oil- or dispersant-treated rats had mixed results with reduced activity at 3 wk and elevated activity at 5 wk compared to controls. However, when the dispersant was combined with oil at 3 wk, a reduction in activity was observed that was similar to that of dispersant alone. One-week nitroreductase activity in the small intestine and cecum was unaffected in the three treatment groups, but elevated activity was observed in the large intestines of animals treated with oil or dispersant. The effect of the combination dose was not significantly different from the control value. Due to experimental error, no 3- or 5-wk NR data were available. By 5 wk of treatment, enterobacteria and enterococci were eliminated from ceca of oil-treated rats. When oil was administered in combination with dispersant, an apparent protective effect was observed on the enterococci and lactose-fermenting and nonfermenting enterobacteria. A more detailed analysis at the species level revealed qualitative differences dependent on the treatment. This study suggests that prolonged exposure of mammals to oil, dispersant, or in combination impacts intestinal metabolism, which ultimately could lead to altered detoxification of oil constituents and coexposed toxicants.