-The present study was designed to investigate testicular effects of Acyclovir [9-(2-hydroxyethoxymethyl)-9H−guanine] in mouse. Swiss albino male mice (N=6/group/dose/sample time) were treated (i. p.) every day with 4, 16, 32, and 48 mg/kg body weight of Acyclovir for 15 d. The testis was examined for histopathological changes and the seminiferous tubular diameter (STD) and epithelial height (SE) were measured. The sperm count, sperm motility and sperm morphology assay in caudae epididymes, and intra-testicular levels lactate dehydrogenase (LDH) and testosterone were estimated on 7 d, In conclusion, Acyclovir is cytotoxic to germ cells inducing the cellular destruction, and reducing the sperm count and motility, and increasing sperm abnormalities. Further, Acyclovir also caused cellular destruction thus releasing LDH from the cells, and affecting the Leydig cell function. All adverse effects of Acycovir are reversible by 70 d, except the sperm count and LDH level, which appear to be affected over an extended period of time at a higher dose-level.