Sexually mature rats were bilaterally or unilaterally vasectomized or subjected to sham-operation at intervals of up to 1 year before autopsy. The gross morphology of the gonads and accessory glands was unaffected by vasectomy although epididymal enlargement was observed in some rats shortly after surgery. Sperm-containing cysts were found near the cut ends of the deferent ducts and in the abdominal cavity. Sexual behaviour was similarly uninfluenced by vasectomy although the animals of one group, vasectomized on the left side only, failed to ejaculate. The significance of this observation is considered.