The stages of the cycle in the rat seminiferous epithelium are illustrated for testes fixed by vascular perfusion and embedded in plastic resins. Improved cellular resolution in plastic sections permitted a clearer demarcation of the stages than in paraffin. Quantitative data are presented to support the recognition of stages, particularly those in transition. Stages IV, V, VII, XI, and XII had the highest frequencies of transitional characteristics. Stage IV was redefined to be more consistent with the occurrence of a high percentage of mitotic figures and to clarify transitions in this stage. Although the resolution of cellular detail was greatly improved with the use of plastics, the thinner sections contained fewer identifying features together within a single tubule cross section and sometimes major characteristics were absent. Therefore, additional characteristics were used for stage classification, such as nuclear diameter and the presence or absence of mitotic figures. A binary decision key is provided to improve consistency among laboratories in the identification of the stages in plastic-embedded testes.