Department of Agricultural Science, University of Oxford, U.K. The duration of the cycle of the seminiferous epithelium is a fundamental parameter of spermatogenesis in mammals (Heller & Clermont, 1964). Its measurement in the field vole (Microtus agrestis) could clarify the way in which seasonal changes in spermatogenesis in this species (Clarke & Forsyth, 1964) are brought about. Furthermore, in our laboratory colonies puberty in M. agrestis occurs at about 6 weeks of age, whereas in the bank vole (Clethrionomys glareolus) it is 5-6 weeks later (Greig, 1968). The duration of the seminiferous epithelial cycle has therefore also been estimated in C. glareolus to determine whether this dissimilarity is associated with a difference in the duration of the cycle of the seminiferous epithelium. As in other species (Roosen-Runge, 1962), spermatogenesis in M. agrestis (Grocock, 1972; Grocock & Clarke, 1975) and C. glareolus (Grocock, 1972) can be divided into 8 stages whose