Antipyrine half-life determined on three to five separate occasions over a period ranging from 3 weeks to 1 year was highly reproducible in each of 20 elderly subjects (12 women and 8 men; mean age, 83 years). The mean within-subject coefficient of variation for antipyrine half-life was 9.6%, whereas the between-subject coefficient of variation was 33.3%. These findings indicate high reproducibility of individual rates of drug metabolism in medically stable elderly subjects.