Background <p>A lack of safety and dosing information on restoring hormone levels in neutered dogs has hindered treatment of spay-neuter syndrome, the diverse set of health and behavior conditions resulting from the loss of gonadal hormones. The purpose of this study was to provide basic information on the use of testosterone treatment in male neutered dogs using a target animal safety study design. Twelve previously neutered dogs were divided into four equal groups, receiving 0 (controls), 1x, 3x, or 5x the standard weekly dose of injectable testosterone cypionate (0.5&#xa0;mg/kg) for 90 days. Bloodwork, including endocrine assays, body condition, prostate health, and qualitative behavioral observations were used to evaluate any changes in health during treatment.</p> Results <p>Testosterone levels increased in a dose-dependent fashion, and luteinizing hormone decreased after a month of treatment in dogs receiving 5x the standard dose. Behavioral measures, Zambelli prostate health score, body condition scores, clinical evaluations, and routine blood hematology and chemistries showed minor variation over time or across groups. Two seizures were documented in a dog from group 3x with previous suspected idiopathic epilepsy.</p> Conclusions <p>The results of the initial safety study indicated that weekly injectable testosterone therapy over a 3-month period was safe and capable of increasing testosterone levels in neutered dogs to within the normal range for intact dogs. The limitations of the study included small group sizes and no long-term follow up. A summary of the risks and recommendations for the use of testosterone restoration in dogs was provided.</p>