The ability of the liver of 5- to 27-month-old male Fischer F344 rats to activate and metabolize aflatoxin B1 (AFB1) was studied. The conversion of AFB1 to mutagenic compounds by liver homogenates was determined using the Salmonella/microsome test. The liver homogenates from 12-month-old rats produced twice as many revertants as liver homogenates from either 5-, 18-, or 27-month-old rats. There was no significant difference in the ability of liver homogenates from 5-, 18-, or 27-month-old rats to activate AFB1. A comparison of the ability of liver homogenates from 5-, 12-, and 20-month-old rats to form AFB1-DNA adducts was made also. Liver homogenates from 12-month-old rats produced twice as many adducts as liver homogenates from 5- or 20-month-old rats. The conversion of AFB1 to water-soluble and chloroform-soluble metabolites by liver homogenates was studied also. An age-related change in the conversion of AFB1 to AFQ1 was noted. The ability of liver homogenates from 5- to 30-month-old female Fischer F344 rats to activate AFB1 was also studied. In contrast to male rats, the activation of AFB1 was lowest by liver homogenates from 12-month-old female rats. Therefore, the ability of the liver to activate AFB1 appears to change with age in Fischer F344 rats, and the age-related change appears to be different for male and female rats.