The Organisation for Economic Co-operation and Development Test Guideline (TG) 488 for the transgenic rodent (TGR) mutation assay recommends two sampling times for assessing germ cell mutagenicity following the required 28-day exposure period: 28 + > 49 days for mouse sperm and 28 + >70 days for rat sperm from the cauda epididymis, or three days (i.e., 28 + 3d) for germ cells from seminiferous tubules (hereafter, tubule germ cells) plus caudal sperm for mouse and rat. Although the latter protocol is commonly used for mutagenicity testing in somatic tissues, it has several shortcomings for germ cell testing because it provides limited exposure of the proliferating phase of spermatogenesis when mutations are fixed in the transgene. Indeed, analysis of sperm at 28 + 3d has generated negative results with established germ cell mutagens, while the analysis of tubule germ cells has generated both positive and either negative or equivocal results. The Germ Cell workgroup of the Genetic Toxicology Technical Committee of the Health and Environmental Sciences Institute modelled mouse and rat spermatogenesis to better define the exposure history of the cell population collected from seminiferous tubules. The modelling showed that mouse tubule germ cells at 28 + 3d receive, as a whole, 42% of the total exposure during the proliferating phase. This percentage increases to 99% at 28 + 28d and reaches 100% at 28 + 30d. In the rat, these percentages are 22% and 80% at 28 + 3d and 28 + 28d, reaching 100% at 28 + 44d. These results show that analysis of tubule germ cells at 28 + 28d may be an effective protocol for assessing germ cell mutagenicity in mice and rats using TG 488. Since TG 488 recommends the 28 + 28d protocol for slow dividing somatic tissues, this appears to be a better compromise than 28 + 3d when slow dividing somatic tissues or germ cells are the critical tissues of interest.