Mastomys (Praomys coucha) is a small murine rodent native in Africa. The present study was undertaken to investigate the developmental time course of mastomys embryos in vivo and the rate of development in vitro. Mastomys embryos developing in vivo exhibited a long 2-cell stage, which lasted from late Day 1 through Day 3. At Day 4, most embryos developed to the 8-cell or morula stages and at Day 5, all embryos reached the blastocyst stage. The attachment of embryos to the endometrium began at Day 5, as revealed by histological examinations. The development in vitro was investigated in different media with three inbred strains and an F1 hybrid. When 1-cell embryos from inbred mastomys were cultured in Whitten's medium (WM), none of them (n=61) developed beyond the 2-cell stage. Embryos of the same strains collected at Day 2 (2-cells) reached the blastocyst stage (46%, 31/67) in the same medium. Meanwhile, 1-cell embryos from F1 hybrid mastomys developed to the blastocyst stage in WM (13%, 13/71), indicating the presence of a strong heterosis effect. CZB medium, which lacks glucose and contains glutamine and EDTA, supported the development of 1-cell inbred embryos to the blastocyst stage (24%, 17/70). Supplementation of glutamine, but not EDTA, to WM stimulated the development of 1-cells to the blastocyst stage (22%, 11/51). These findings indicate that the development of mastomys embryos in vitro is both strain and medium dependent and that glutamine may play a crucial role in early development of mastomys embryos.