The aloe ingredients responsible for physiological effects and the concentrations required to exert their biological activities are not fully understood. This study compares the anti-inflammatory effects of aloin and aloe-emodin with other polyphenols. Our results demonstrated that aloe-emodin dose-dependently inhibited inducible nitric oxide synthase (iNOS) mRNA expression and nitric oxide (NO) production at 5-40 microM. In addition, the levels of cyclooxygenase-2 (COX-2) mRNA and prostaglandin E2 (PGE2) production were suppressed by 40 microM aloe-emodin. Aloin also suppressed the production of NO at 5-40 microM, although it did not suppress PGE2 production. The present results indicate that aloin and aloe-emodin possibly suppress the inflammatory responses by blocking iNOS and COX-2 mRNA expression. The anti-inflammatory effect of aloe-emodin was comparable to that of kaempferol and quercetin, indicating aloe-emodin as a possible key constituent responsible for the anti-inflammatory activity of aloe.