The rabbit embryo is able to develop from the two-cell stage to the morula in a medium containing an amino-nitrogen source as the only metabolizable component. This amino-nitrogen source can be bovine serum albumin, oxidized glutathione, or certain single amino acids such as glutamine, proline or alanine. Although a requirement for a non-nitrogenous energy source could not be demonstrated during this period, a beneficial effect of pyruvate and lactate was shown. The optimum concentration for pyruvate was approximately 10 −4 m and for lactate was 1 to 3×10−2m.