Neuroendocrine changes initiate female reproductive function but little is known about the mechanisms that determine the onset of puberty. The authors identify an epigenetic mechanism of transcriptional repression that regulates the timing of female puberty in rats. They found that increased methylation of the promoter region of two genes encoding Polycomb group family members reduced their expression at the onset of puberty, allowing the expression of Kiss1, which controls the release of gonadotropin-releasing hormone from neurosecretory neurons in the hypothalamus.