1-Nitropyrene (100 mg/kg body wt.) was administered by gavage to conventional and germfree Fischer 344 rats. 1-Aminopyrene was detected in the feces of conventional rats (5-6% of the dose) but not in the feces of germfree rats. Conjugates of 1-aminopyrene were not detected in the feces of germfree rats and neither 1-aminopyrene nor its conjugates were found in the urine of either conventional or germfree rats. Since reduction of 1-nitropyrene to 1-aminopyrene is an activation process, the results indicate that intestinal microflora are important in the metabolic activation of 1-nitropyrene in vivo in the rat. Human intestinal microflora also reduced 1-nitropyrene to 1-aminopyrene.