The expression of casein genes is under the control of several hormones of which the most important are prolactin, glucocorticoids and progesterone. In pseudopregnant or mid-pregnant rabbit having partially developped but inactive mammary gland, prolactin induces casein synthesis. The phenomenon is accompanied by an accumulation of casein mRNAs and by a stimulation of their translation. The accumulation of casein mRNAs results from an acceleration of the transcription of the corresponding genes and from a stabilization of the mRNAs. These prolactin effects are amplified by glucocorticoids which are not per se inducers and they are inhibited by progesterone. The essential action of prolactin and glucocorticoids can be obtained in cultured mammary explants and epithelial cells. This induction is accompanied by a transformation of the mammary cell in which are accumulated ribosomes and membranes involved in milk synthesis and exportation. This transformation is favoured by prolactin and inhibited by progesterone. Hence, the abundant milk secretion is triggered only after parturition when the predominence of progesterone is reversed in favour of prolactin. Prolactin incubated with mammary membranes promotes the formation of a factor capable of accelerating beta-casein gene transcription when added to isolated mammary nuclei. This factor is formed only by lactogen hormones and from prolactin receptor containing membranes. The information contained in the factor seems to be understood only by prolactin target genes. The generation of the factor can be provoked by anti-prolactin receptor antibodies and it is inhibited by tubulin binding drugs such as colchicine. The molecule exhibiting prolactin-like activity has a small molecular weight, it is thermostable and inactivated by trypsin. The stimulation of beta-casein gene transcription is abolished when the factor is incubated with nuclei in the presence of phosphatase inhibitors. These facts suggest that prolactin after its binding to its peripheral receptors triggers the release of a small peptide which migrates to nuclei where it activates the transcription of the prolactin target genes through a dephosphorylation of nuclear proteins. This small peptide is a good candidate to the prolactin intracellular relay.