G207, a replication-competent herpes simplex virus type 1 (HSV-1) virus, has been previously shown to be effective against human prostate cancer xenografts in mice. This study assesses its safety in the prostate of two animal models known for their sensitivity to HSV-1. BALB/c mice were injected intraprostatically with either HSV-1 G207 or strain F and observed for 5 months. None of the G207-injected animals exhibited any clinical signs of disease or died. However, 50% of strain F-injected mice displayed sluggish, hunched behavior and died by day 13. Histopathologically, the G207-injected prostates were normal whereas strain F-injected prostates showed epithelial flattening, sloughing, and stromal edema. Four Aotus nancymae monkeys were also injected with G207 intraprostatically and observed short term (up to 21 days) and long term (56 days). Safety was assessed on the basis of clinical observations, viral biodistribution, virus shedding, and histopathology. None of the injected monkeys displayed evidence of clinical disease, shedding of infectious virus, or spread of the virus into other organs. Except for minor histological changes unrelated to the study, no significant abnormalities were observed. These results demonstrate that G207 can be safely inoculated into the prostate and should be considered for human trials for the treatment of prostate cancer.