The lateral scent organs ofArvicola terrestris(L.) are basically similar in construction to those ofA. scherman(Shaw), and differ in that they do not express long cylinders of secretion at the end of the breeding season, and that frequent epidermal vesicles are observed. During the year the organ of the male undergoes a cycle of activity with a peak in secretory activity shortly following the peak in the cycle of testis weight. Bilateral castration causes a marked involution of the organ in the male, which can be reconstituted by the administration of exogenous testosterone.