Seasonal variations in biomass and alkaloid contents of goldenseal (Hydrastis canadensis) were investigated. Five-year-old plants gave 5× the yield of roots and rhizomes of two-year-old plants, and summer growth gave significant increases in root biomass but not rhizomes. Berberine contents of roots plus rhizomes did not vary significantly and were > 3.4% in all samples. Hydrastine contents of 5 y roots plus rhizomes showed significant seasonal variation. These variations were due to significant changes in the hydrastine contents of the roots (1.3–1.9%), but not the rhizomes (2.2–2.8%). Goldenseal leaves plus stems had lower contents of hydrastine (0.4–0.8%) and berberine (1.0–1.5%).