The metabolic turnover, absolute oral bioavailability, clearance, and volume of distribution for β-sitosterol were measured in healthy subjects. [(14)C]β-Sitosterol was used as an isotopic tracer to distinguish pulse doses from dietary sources and was administered by both oral and intravenous routes. The administered doses of [(14)C]β-sitosterol were in the region of 3 to 4 μg, sufficiently low as not to perturb the kinetics of β-sitosterol derived from the diet. Because the plasma concentrations of [(14)C]β-sitosterol arising from such low doses were anticipated to be very low, the ultrasensitive isotope ratio analytical method of accelerator mass spectrometry was used. The limit of quantification for [(14)C]β-sitosterol was approximately 0.1 pg/ml, the oral absolute bioavailability was just 0.41%, clearance was 85 ml/h, volume of distribution was 46 L, and the turnover was 5.8 mg/day. Given the steady-state concentrations of β-sitosterol (2.83 μg/ml), then the dietary load was calculated to be approximately 1400 mg/day.