In the first part of the investigations the interaction between oestradiol-17 beta, progesterone and prolactin on the formation of the lobuloalveolar structure of the mammary gland was studied in 21 nulliparous intact postpubertal ewes. Tissue specimens of the mammary gland were evaluated by light microscopical examination and prolactin was measured in blood samples. Combined treatment with oestradiol and progesterone induced physiological development of the lobuloalveolar structure of the mammary gland in the presence of prolactin. Suppression of prolactin by bromocriptine prevented this effect. Treatment with progesterone alone was not effective and revealed histological structures comparable to the controls. The interaction of oestradiol-17 beta with prolactin appears to be a main trigger for mammary gland development. In the second part of the investigations the effect of bromocriptine treatment during pregnancy on mammary gland development was studied in primigravid ewes and heifers. Mammary gland specimens were obtained for light and electron microscopical evaluation in ewes and heifers 5 and 10 days respectively before the expected date of parturition. Concentrations of prolactin and ovine placental lactogen were determined by radioimmunoassay. Bromocriptine treatment suppressed prolactin concentrations in ewes to minimal values but did not affect ovine placental lactogen concentrations. The histological evaluation revealed almost comparable stages of mammogenesis in both ewes and heifers. This effect can be attributed, at least in ewes, to the action of placental lactogen on mammogenesis. Placental lactogen might therefore be of physiological relevance during mammogenesis.