Six diets were fed to weanling rats and the in vitro caecal microbial nitroreduction rates of metronidazole were determined. The sensitivity of this metabolic pathway to diet was demonstrated by the observed 20-fold range in reaction rate constant. Methods were devised to investigate the mechanism of this dietary effect. It was concluded that indigestible dietary components affected primarily both the weight of caecal contents and the metabolic activities of the microorganisms per unit weight of caecal contents. It appeared that the actual reaction rate in the caecum might be 35% higher than that determined by the standard assay procedure that involves sample dilution. However, the different soluble materials in the caeca derived from rats fed different diets had no significant secondary regulatory effects on the rate constant.