AbstractDuring the breeding season from January to June, female wallabies which are suckling a young animal in the pouch may carry a dormant embryo (lactational quiescence). Removal of the pouch young during this period results in a resumption of embryo development. In the latter half of the year, the embryo will not reactivate after removing the suckling young (seasonal quiescence). In this situation, development resumes spontaneously in late December or may be induced prematurely by progesterone treatment. The response of the genome of quiescent macropod blastocysts was studied during the early period of growth. Changes in the transcriptional activity of the embryo cells were measured by assay for endogenous RNA polymerases. Embryos actively synthesized RNA during both lactational and seasonal quiescence. Termination of seasonal quiescence resulted in increases in RNA polymerase activities within the nucleolus and nucleoplasm of the cell. This occurred on the day following the summer solstice, December 22, 1974, in animals captured in the wild, or within 48 hours of administration of progesterone. Embryos which were induced to resume development during the breeding season also showed increases in nucleolar and nucleo‐plasmic polymerase activities within five days of removal of suckling young from the pouch. In all situations, the response of the nucleolar enzymes was greater than that of the nucleoplasmic enzymes. This is in agreement with other observations of the regulation of gene activity in growth‐stimulated ce