Inner cell masses (ICMs) were dissected from 3 1/2- and 4 1/2-day mouse blastocysts and inserted into empty zonae before transfer to the oviducts of pseudopregnant mice. The ICMs survived in the oviduct for at least 2 days with little evidence of reduction in cell number. After 1 day, isolated 3 1/2-day ICMs were still viable, since they could form chimaeras when injected into 3 1/2-day blastocysts. However, no evidence of trophoblast formation could be detected in any of the isolated ICMs, despite exposure of some of their cells to 'outside' conditions. This is further evidence that ICM cells, although not overtly differentiated, are determined by 3 1/2 days. Although no trophoblast was formed, 3 1/2-day ICMs formed an outer endoderm layer after 1 day in the oviduct, as judged by light and electron microscopical evidence. It is suggested that cell position may be important in endoderm differentiation.