PIP: The activity of 3 enzymes related to the bioactivation of toxic compounds and the development of cancer--cytochrome P450 IA2, N-acetyl transferase (NAT), and xanthine oxidase (XO)--can be measured from the ratios of formed metabolites excreted into urine. In the 3 experiments that comprised this study, subjects received at least 1 cup of coffee 2- 6 hours before spot urine samples were taken. The subjects included 335 healthy male and female volunteers who provided information on tobacco, caffeine, and broccoli intake in the preceding 2 weeks, 23 healthy men who exercised 8 hours/day for 30 days, and 9 subjects whose diet included green beans and broccoli. As expected, the ratio reflecting P450 IA2 activity was 66% and 70% higher, respectively, in men and women who smoked at least 10 cigarettes/day compared to male and female nonsmokers. The XO ratio also was significantly increased in smokers. 30 days of vigorous physical exercise increased the P450 IA2 ratio by 50% and the XO ratio by over 100%. Broccoli induced a 19% increase in P450 IA2 activity, while pregnancy and oral contraceptive use reduced this ratio by 29% and 20%, respectively. Since these ratios appear to yield reliable indicators of enzyme activity, prospective studies of their association with cancer development are recommended.