We have shown in the accompanying paper that a developmental transition occurs at the midblastula stage (cleavage 12) in Xenopus embryos, and that this midblastula transition (MBT) is apparently initiated when the ratio of nucleus to cytoplasm reaches a critical value. One manifestation of this transition is the onset of transcription. We show here that a plasmid containing a cloned gene coding for a yeast leucine tRNA comes under developmental control when injected into cleaving eggs. In pre-MBT eggs this plasmid is transiently transcribed and then becomes inactive; however, it becomes transcriptionally active again at the MBT. This pre-MBT suppression of transcription can be reversed by addition of competing DNA. The amount of DNA needed to induce premature transcription is equal to the amount of nuclear DNA present after 12 cleavages (24 ng), suggesting that the MBT is triggered by the DNA through titration of suppressor components present in the egg.