The main objectives of this study were to determinewhether microfilariae in blood collected at later time pointsduring their decline after doxycycline treatment at a rela-tively high dosage would regain their ability to develop toL3that were capable of continuing their development ina 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 sched-ule for doxycycline used in this study is widely used byveterinarians in adulticide therapy for heartworm-positivedogs (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.