Early somite (2-4) mouse embryos were explanted and then maintained in culture for 24 or 48 h intervals. Various types of media were tested and it was determined that rat serum supported normal growth over a period of 48 h, based on total protein analysis and histological comparisons with in vivo specimens. Other media including fetal calf serum and fetal calf serum and Waymouth's (1:1) supported some growth, but did not equal the success of using rat serum alone. During the 48 h culture period in rat serum, embryos developed to stages indistinguishable from embryos maintained for a similar time in vivo.