Cells at various stages of spermatogenesis show remarkable differences in their sensitivities to toxic and mutagenic agents. For accurate analyses of toxicity and mutagenicity, the most sensitive cell types should be obtained for studies soon after treatment before development of disturbing secondary alterations. The transillumination phase contrast microscopic technique has proved to be useful for this purpose. It allows recognition of rat and mouse seminiferous tubules in living unstained conditions for accurate selection of the desired stages of the cycle of the seminiferous epithelium for morphological and biochemical studies. Specific cell damage can be frequently recognized by transillumination only, whereas in most cases phase contrast microscopy is useful in screening the early stage-specific toxic alterations. A summary is presented of the results obtained by treatment with heat and with anticancer drugs. A meiotic micronucleus method has been developed for direct estimation of the severity of mutagenic insult on rat spermatogenesis. The segment that contains the second meiotic division and the early postmeiotic cells (stages XIV and I) is selected for study. Chromosome damage induced by mutagens may result in the formation of acentric fragments that after meiotic divisions are seen in early spermatids as separate micronuclei. They can be quantitated in squash preparations; the sensitivity of this method is comparable with that of the meiotic metaphase scoring. Recently, an in vitro technique has been developed for stage and cell specific measurements of replicative and repair syntheses of DNA during rat spermatogenesis after mutagen treatments.