To implement a new macrofilaricide, treatment of heartworm infection or disease in dogs was checked in all the clinical situations ie from subclinical to severe disease. After preliminary toxicity and efficacy models on experimentally infected dogs, in addition, to the reference posology (2.5 mg of melarsomine (RM 340)/kg twice, 24 h apart by deep IM injection) a more practical program for vet practitioners was suggested (2.2 mg/kg twice, 3 h apart) using modelization of the pharmacokinetic data. The two treatments were equivalent as shown on models with experimental infection of dogs, critical tests on naturally infected dogs and clinical trials in veterinary practice. We advise using specific and well adapted therapeutic programs for each of the clinical classes (class 1: subclinical, class 2: moderate, class 3: severe). The safety margin is respectively x 3 or x 2.5 in contrast with thiacetarsamide which, being hepatotoxic, has no safety margin, and sometimes is nephrotoxic at the recommended dose. RM 340 is fully effective on D immitis adults (even on young ones of 7 months old) and L5 immatures (4 months old) when thiacetarsamide is poorly effective on 7 months or ineffective on 4-month-old parasites. Clinical trials in veterinary practice showed that the programs are well adapted to many clinical situations. The product is effective, relatively safe and easy to handle by IM injection. Preliminary results show its possible use as tactical treatment (2.2 mg/kg twice, 3 h apart) twice a year in mid August and December-January to prevent heartworm disease.