Dairy heifers were submitted to high or moderate plane of nutrition before and during first gestation. High plane of nutrition did not increase milk production but negatively affected reproduction and health. In addition, high plane of nutrition decreased growth hormone (GH) concentration and increased prolactin concentration during gestation. There was no effect of previous plane of nutrition on peripartum concentrations of hormones. Milk production was positively correlated with GH concentrations, but was negatively correlated to concentrations of insulin-like growth factor-1(IGF-1) concentrations postpartum. / The effect of GH administration on mammary vasculature and on paracrine secretion of growth factors was investigated in lactating and non-lactating dairy cows. The injection of GH induced a linear increase of IGF-1 concentration in blood and mammary primary lymph, but did not affect concentrations of prostaglandin E$ sb2$ (PGE$ sb2$) concentrations. However, the rate of IGF-1 increase was slower in lymph than in blood, suggesting that blood is the source of most lymph IGF-I. Therefore, GH did not seem to affect paracrine secretion of IGF-1 and PGE$ sb2$ by the bovine mammary gland. Growth hormone administration induced a gradual increase in mammary blood flow in lactating cows while it remained constant in non-lactating animals. Changes in lymphatic, but not blood, concentrations of a stable metabolite of the vasodilatator prostacyclin I$ sb2$ parallelled the changes in blood flow. This indicates that local secretion of this vasodilatator may be responsible for the effect of GH on mammary blood flow. / Mitogenic activity of mammary primary lymph samples taken at different physiological stages was evaluated on mammary epithelial and mammary fibroblast cell lines. The results indicate that the mitogenic activity of lymph in epithelial cells was not related to physiological stage but to PGE$ sb2$ concentration. In fibroblasts, mitogenic activity of lymph was not related to physiological stage or to the content in any of the growth factors assayed.

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