An ethanolic extract of the flowering meristems of worm wood, Artemisia nilagirica was allowed to evaporate. The residue, thus obtained, was administered orally on 4 pariah dogs naturally infected with Dirofilaria immitis at 10 mg/kg/day for 15 days and then at 20 mg/kg/day for the next 15 days. Two homoeopathic potencies of the A. nilagirica extract, called Cina 200 and Cina 1000, were obtained commercially and administered orally at 0.1 ml/dog/day for 30 days on two separate batches, each consisting of 4 dogs. Blood was sampled from the dogs before treatment and on day 15, 30, 45 and 75 following the treatment. A. nilagirica extract (Cinaθ) was diluted with 90%ethanol1 : 100 and shaken by 10 manual strokes to prepare the 1st potency, called Cina 1. All subsequent potencies were prepared by mixing 1 part of the preceding potency with 99 parts of 90% ethanol and giving the mixture 10 manual strokes. Cinaθ, Cina 200 and Cina 1000 reduced microfilarial densities in treated dogs by 78.38, 63.06 and 71.40%, respectively on day 30. There were 57.13, 42.44 and 64.20% reduction on day 75. No apparent toxic effect was observed in the treated dogs. Electronic spectra of Cinaθ, Cina 200 and Cina 1000 showed comparable absorbance with the latter two giving a blue shift. Cinaθin CCl4 showed a red shift suggesting molecular complexation and charge transfer (CT) interaction between aqueous ethanol and compounds of A. nilagirica. CT was further evidenced by the NMR spectra of the deuterium nuclei of Cinaθin 90% ethanol. NMR spectra of Cinaθ, Cina 200, Cina 1000 and 90% ethanol indicated a change in the solution structure of Cina 200 and Cina1000. This altered solution structure is thought to be responsible for inducing immune reaction of the hosts against the parasite.