The development of the early 2-cell mouse embryo to the late 2-cell stage is marked by the appearance between 23 and 26 h post-insemination of a complex of polypeptides of mol. wt. approximately 67 K. Addition of alpha-amanitin between 18 and 21 h post-insemination prevents or reduces the subsequent appearance of these polypeptides. Addition of alpha-amanitin after 21 h does not obviously affect the appearance of the approximately 67 K polypeptides. A major change in synthetic profile occurs between 29 and 32 h post-insemination involving many polypeptides. Addition of alpha-amanitin to 2-cell embryos prior to 29 h post-insemination prevents the appearance of the new polypeptides observed during this major change but does not prevent the disappearance of the old polypeptides. In contrast, addition of alpha-amanitin after this time does not affect the appearance of the new polypeptides. This result, together with other evidence presented, suggests that during the 2-cell stage the embryonic genome shows transcriptional activity in two phases at 18-21 and 26-29 h post-insemination, that these transcripts are utilized soon after their synthesis, and that most maternal transcripts used before the second phase of embryonic transcription become ineffective soon afterwards.