We have studied the role of the nucleo-cytoplasmic ratio in the early development of Drosophila, using mutants and experimental manipulations that alter nuclear density. Haploid embryos produced by either maternal or paternal effect mutations compensate for haploidy by an extra nuclear division during the syncytial blastoderm stage. Decreasing the nucleo-cytoplasmic ratio in wild-type embryos by ligation can cause a similar extra blastoderm division. Conversely, increasing this ratio can cause the omission of a blastoderm division. The duration of mitotic cycles is affected by the nucleo-cytoplasmic ratio four cycles before the terminal blastoderm division. Transcription patterns in haploid embryos indicate that transcriptional activation is not directly controlled by the nucleo-cytoplasmic ratio, but may be an effect of the lengthening of interphase periods.