Indole-3-carbinol (I3C), found in cruciferous vegetables, has been shown to suppress tumorigenesis at estrogen-responsive sites. This effect may be mediated through modification by I3C of the cytochrome P450 (CYP) complement and activities leading to estrogen detoxication. In this study, we examined the effects of 4- and 10-day treatments of female Sprague-Dawley rats with I3C at 5, 25, and 250 mg/kg body weight, administered by oral gavage, on CYP mRNA expression in the liver and mammary gland, CYP-dependent activities, and the metabolism of 17beta-estradiol (E2) and estrone (E1) by liver microsomes. The mRNA transcripts for hepatic CYP1A1, 1B1, and 2B1/2 and mammary CYP1A1 were up-regulated after treatment with I3C at 250 mg/kg. However, the level of expression of CYP1B1 in the liver was lower than that of other CYPs. In the mammary gland, CYP1B1 mRNA levels were unaltered by treatment and similar to those of I3C-induced CYP1A1. Hepatic P450 probe activities indicative of induction of CYP1A1, 1A2, and 2B1/2 were increased by I3C in a dose-dependent manner. Treatment with I3C at 250 mg/kg increased the capacity of liver microsomes to metabolize E2 to 2-OH-E2, 2-OH-E1, 6alpha-OH-E2, 6beta-OH-E2, estriol, and 15alpha-OH-E2, and E1 to 2-OH-E1, 2-OH-E2, 6(alpha+beta)-OH-E1, and 6alpha-OH-E2. The magnitudes of increases of CYP-dependent activities and rates of estrogen metabolite formation achieved with I3C at 250 mg/kg were smaller after ten than four treatments. The increased rates of formation of 6alpha-OH-E2, 6beta-OH-E2, and 15alpha-OH-E2 from E2 were also detected after treatment with I3C at 25mg/kg, and, except for increased 6beta-OH-E2 from E2, no other changes in E2 or E1 metabolism occurred after treatment with I3C at 5mg/kg. The data indicate that alterations in the CYP complement and, thus, metabolite composition from E2 and E1 are I3C dose- and treatment duration-dependent, and suggest that potential biological activity of I3C administered at low doses to rats may not involve changes in estrogen metabolism.