SummaryThis paper, which is published in two parts, reviews the literature pertaining to antiviral chemotherapy of viruses of veterinary importance. While early reports in the 1970s referred to the chemotherapy of a number of different RNA and DNA viruses, there' was considerable focus in the 1980s, initially on herpesviruses and latterly on retroviruses, and particularly in cats. Details are given of the successful treatments of FeLV and FIV, which have been used as animal models for HIV therapy. 'Therapy of equine, canine, bovine, porcine, avian, and fish diseases is also considered. The high costs of developing and registering a new chemical entity, especially for food species in which extensive toxicity/residue data are required, is the main reason why specific antiviral compounds are not currently available for veterinary use, although some non-specific immune modulators are now emerging. Concurrent availability of appropriate diagnostic tools is a prerequisite for successful veterinary antiviral chemotherapy, as is a greater understanding of the pathogenesis of virus infections in animals and the development of more sophisticated means of drug delivery, appropriate to both food animal species and companion animals (dogs, cats, and horses). Additionally, antiviral agents are valuable as research tools per se, as opposed to solely as chemotherapeutic agents. Part 1 covers the feline virus diseases, while part 2 includes the other viruses~f veterinary importance, in dogs, horses, cattle, pigs, birds, and fish.