Activity of cytochrome P450 3A4 (CYP3A4), the most abundant human P450 isoform and responsible for metabolizing approximately half of all therapeutic agents, has been speculated to vary during the menstrual cycle. This investigation evaluated CYP3A4 activity during the menstrual cycle, using midazolam clearance as a metabolic probe. Midazolam (1 mg i.v.) was administered to nonsmoking, nonpregnant female volunteers (N = 11, age 26 +/- 5 years) with normal menstrual cycles on three separate occasions during the same cycle: days 2 (menstrual phase), 13 (estradiol peak), and 21 (progesterone peak). Venous plasma midazolam concentrations were determined by gas chromatography-mass spectrometry. Midazolam clearance was determined by noncompartmental and compartmental analysis. Midazolam plasma disposition did not differ between phases of the menstrual cycle. There was no significant difference in any measure of midazolam clearance. Noncompartmental clearances (mean +/- SD) were 7.36 +/- 2.73, 6.34 +/- 3.59, and 6.23 +/- 2.04 ml/kg/min, respectively, on days 2, 13, and 21 of the menstrual cycle. These results suggest no difference in hepatic CYP3A4 activity on menstrual cycle days 2, 13, and 21. Consideration of menstrual cycle variability in the metabolism of CYP3A4 substrates does not appear indicated in the dosing or design of clinical trials.