Rat 1-cell embryos fertilized in vivo or in vitro show a complete developmental block at the 2- to 4-cell stage in conventional culture media. To develop a culture medium supporting early embryonic development in rats, we examined the effects of some chemical and physical factors on the development of rat 1-cell embryos. The results indicated that (1) the developmental block is caused by phosphate, (2) glucose and amino acids are necessary, (3) the optimal osmolarity is 244–246 mOsm, and (4) the presence of bovine serum albumin and relatively high (110.0–130.0 mM) NaCl concentrations in the early stages preceding pronucleus formation is necessary. On the basis of these results, novel culture media designated as modified rat 1-cell embryo culture medium (mR1ECM) and mR1ECM for in vitro fertilization (mR1ECM-IVF) were developed. In cultures using these media, 90% or more of rat 1-cell embryos fertilized in vivo or in vitro successfully developed to the blastocyst stage in vitro.