Acute treatment of rheumatoid rats with an extract from the roots of Gentiana macrophylla (Gentianaceae) produced a significant inhibitory effect on rheumatoid arthritis (RA). When rats were administered the Gentiana macrophylla extract orally at a daily dose of 100 mg/kg body weight, the prostaglandin E(2) (PGE(2)) levels in the inflammatory tissues, sole thickness, and ankle circumferences of feet were significantly decreased. The anti-inflammatory activity observed in Gentiana macrophylla is comparable to that observed in prednisone. These observations suggest that Gentiana macrophylla displays considerable potency in anti-inflammatory action and could be used as an anti-inflammatory agent in the control of inflammation of rheumatoid arthritis.