Laboratoire de Physiologie de la Lactation, Institut National de la Recherche Agronomique, 78350-Jouy-en-Josas, France (Received 23 December 1974) Hormonal induction of milk synthesis by the ruminant mammary gland in organ culture has received little attention. M. A. Cannata, C. Delouis, C. Jeulin & R. Denamur (unpublished observation) demonstrated the lactogenic effect of insulin + cortisol + prolactin in vitro on ewe mammary tissue with lobulo-alveolar development at the time of explantation, but there are no data on the induction of milk secretion in bovine mammary tissue cultured on a synthetic medium. Therefore, we have studied the culture conditions necessary to maintain bovine mammary tissue and the combination of hormones needed to induce milk secretion. Biopsy samples were obtained from 2-year-old French Friesian heifers at various times during the oestrous cycle, after i.m. injection of the tranquillizer, xylazine (Rompun, Bayer; 2% solution, 1·5 ml/100 kg body weight). Expiants (1 mm3) were