The main objectives of this study were to determine whether microfilariae in blood collected at later time points during their decline after doxycycline treatment at a relatively high dosage would regain their ability to develop to L3 that were capable of continuing their development in a dog and to assess the effects of this treatment on post treatment microfilaria counts and antigen scores and adultworm counts at necropsy. The dosage and treatment schedule for doxycycline used in this study is widely used byveterinarians in adulticide therapy for heartworm positive dogs (i.e., 10 mg/kg body weight administered twice dailyfor 1 month). This present report describes partial resultsfrom a complex study, which is still ongoing.