Pituitary growth hormone (GH), prolactin (PRL), placental lactogen (PL), insulin-like growth factor 1 (IGF-1) and GH-binding protein (GHBP) in plasma were determined in 12 women with gestational diabetes mellitus (GDM) and in 12 healthy pregnant women during a breakfast meal tolerance test. The women with GDM showed higher prepregnancy weight, body mass index (BMI), basal levels of glucose, insulin, and C peptide compared to the pregnant controls. No difference was found between the two groups in pituitary GH, PRL, PL and IGF-1 levels. Plasma levels of GHBP were higher in the women with GDM compared to pregnant controls. In all women there was an inverse correlation between PL and pituitary GH as well as between PL and GHBP, suggesting that PL inhibits pituitary GH secretion. A positive correlation between GHBP and BMI was found in all women, and the higher BMI in the GDM women seemed to be the cause of the higher GHBP levels in this group. In all women IGF-1, an indicator of the secretory activity of lactogenic hormones as well as of nutritional state, showed a positive correlation with the birth weights of the infants and was equally indicative in both groups.