CONTEXT: Byrsonima crassifolia is a tropical tree, commonly known as nance and distributed widely in Mexico and Central and South America. Since pre- Hispanic times, the seeds of the fruits have been used in folklore medicine as an anti-inflammatory; however, currently no researchers have examined its potential pharmacological properties in scientific studies. OBJECTIVE: This study investigated the anti-inflammatory activity of extracts obtained with the solvents n-hexane, chloroform, and methanol from seeds of B crassifolia. DESIGN: The research team induced edemas in Wistar rats with 12-O-tetradecanoylphorbol (TPA), formaldehyde, carrageenan, and histamine to study the anti-inflammatory activity of the three organic extracts of seeds from B crassifolia. The team also used the cotton-pellet granuloma method to induce edemas in Wistar rats and study the inhibitory effect of the three extracts from B crassifolia. Finally, the team examined the participation of the nitric oxide (NO) system in the anti-inflammatory activity of the hexane extract of nance seeds (NS), diclorofenac, and L-NAME as well as the effects of L-arginine and D-arginine on the antiinflammatory actions of the compounds. SETTING: This research was conducted in the Laboratory of Research of Natural Products, School of Chemical Engineering, National Polytechnic Institute (IPNESIQIE) and Department of Biotechnology and Bioengineering, Cinvestav-IPN, Av. IPN 2508, Col. San Pedro Zacatenco, Mexico D.F., CP 07360, Mexico. OUTCOME MEASURES: The research team measured the edema that the solvents caused, either in the ears of rats for tetradecanoylphorbol or in the paws for formaldehyde, carrageenan, and histamine. To study the antiproliferative effects of the extracts after implantation of the cotton-pellet granuloma, the team determined the wet and dry weights of the pellets, after drying at 70°C for 1 hour in the second case. To study the participation of the NO system in the anti-inflammatory activity of the hexane extract of NS, diclofenac, and L-NAME, the research team measured paw edema. RESULTS: Among the extracts tested, NS showed the most significant anti-inflammatory activity. That extract decreased the paw edema that carrageenan, formaldehyde, histamine, and cotton pellet-induced, either by oral or topical administration at doses of 200 mg/kg, with 31%, 66%, 83%, and 58.2% inhibition respectively. In addition, NS inhibited the ear edema that TPA induced by 62%. Methanol and chloroform extracts produced a small effect, so the team does not present the results in this article. L-arginine, a precursor of NO, significantly inhibited the anti-inflammatory effects of NS and L-NAME, an anti-inflammatory drug, on mouse paw edema, but D-arginine did not. In contrast, neither D-arginine nor L-arginine inhibited the anti-inflammatory effects that diclofenac produced. These results indicate that the anti-inflammatory effect of NS on mouse paw edema occurs via the inhibition of NO production, as does the anti-inflammatory effect of L-NAME but not the anti-inflammatory effect of diclofenac. The anti-inflammatory activity of NS was comparable to standard anti-inflammatory drugs such as indomethacin, dexamethasone, and sodium diclofenac. CONCLUSIONS: The hexane extract from seeds of B crassifolia exhibited significant anti-inflammatory activity in both acute and chronic inflammatory models with a partial contribution of inhibitory actions on some cellular inflammatory responses. The anti-inflammatory mechanism of NS may be related to the other isoform (iNOS).