Diethylcarbamazine (DEC) administered orally to jirds and striped mice (300 mg/kg for five days) was effective against the microfilariae of Monanema globulosa in experimental infections. Based on the small numbers of animals used in this trial, it is questionable whether the drug had significant activity against the adult worms in pulmonary arteries. Densities of microfilariae in the ears (mf/mg) fell by 80-100% of pretreatment levels over a four week period following treatment. There was no apparent action of the developing embryos in utero, and in some rodents densities of microfilariae in the skin returned to 73-88% of pretreatment levels within 70 days. No microfilariae were detected in the blood or urine during treatment. Twenty-four hours after the first dose of DEC, many of the microfilariae in the dermis became surrounded by a local infiltration of neutrophils, eosinophils and histiocytes. Cellular reactions were accompanied by an acute pruritus in rodents, as indicated by scratching and distress exhibited soon after treatment: No reactions to microfilariae were seen in untreated rodents. Similarities between this response and the Mazzotti reaction induced by DEC in human onchocerciasis indicate that M. globulosa may be of value as a rodent model to study adverse drug reactions in Onchocerca volvulus infections.