Prolactin is a very versatile hormone in terms of biological actions. In swine, its essential role for lactogenesis was clearly demonstrated when using the dopamine agonist bromocriptine to inhibit prolactin secretion and therefore lactation. Yet, its implication for the maintenance of milk production throughout lactation in sows was only recently demonstrated. Inhibition of prolactin at various stages of lactation drastically decreased or abolished piglet weight gain during the treatment period, thereby demonstrating that prolactin is essential for both onset and maintenance of milk production in sows. The role of prolactin for galactopoiesis seems as important from the first to the fourth week of lactation. Yet, injections of recombinant porcine prolactin to lactating sows did not increase milk yield, growth rate of piglets or mammary development. However, the mean duration of intervals between nursings was reduced. Exogenous prolactin may have had negligible effects in lactation since virtually all prolactin receptors were already saturated prior to treatment. Prolactin is also required for mammary development in various species but its possible role for mammogenesis in pigs was never investigated. A project where gilts received bromocriptine or placebo from days 70 to 110 of gestation was therefore undertaken. Mammary gland development was negatively affected by bromocriptine treatment; this effect may not only have been due to lower concentrations of prolactin but also to a decreased number of prolactin receptors in the mammary gland. Overall, these recent findings clearly demonstrate that a certain threshold of prolactin is essential for normal mammary gland development in gilts and maximum expression of milk yield potential in sows. It also appears that saturation of prolactin receptors in the mammary gland may be a limiting factor for optimal mammogenesis and galactopoiesis.