The expression of the mouse metallothionein-I (mMT) promoter/human growth hormone (hGH) fusion gene in transgenic mice leads to female sterility and major alterations in the function of the hypothalamic-adenohypophyseal system. These alterations include increases in median-eminence norepinephrine turnover and plasma LH levels, and a decrease in plasma prolactin levels in intact males, and an increase in median-eminence dopamine turnover combined with the suppression of plasma prolactin levels in ovariectomized females. To further characterize these changes and to determine whether they are due to the lactogenic or somatotropic activity of hGH, we have studied hypothalamic and pituitary function in transgenic mice expressing mMT/hGH, mMT/hGH.B 'variant', or mMT/bGH fusion genes. In males, the expression of the hGH.B gene was associated with a reduction in pituitary prolactin release in vitro and an increase in LH response to LHRH stimulation, while the bGH transgene did not affect any of the examined parameters of LH and prolactin release. Median-eminence norepinephrine turnover was increased in each of the three lines of transgenic males, while median-eminence dopamine turnover was reduced only in animals expressing the hGH.B gene. In ovariectomized females, plasma LH was suppressed by hGH variant expression, while median-eminence norepinephrine turnover was suppressed in both hGH.B and bGH animals. The turnover of dopamine was increased in the median eminence of females expressing either of the human genes (hGH or gHG.B) and reduced in the median eminence of ovariectomized bGH females. We conclude that the hGH.B gene is weakly lactogenic in mice, and that the chronic stimulation of either GH receptors (by bGH) or both GH and prolactin receptors (by hGH or hGH.B) can lead to profound alterations in the metabolism of hypothalamic neurotransmitters and pituitary hormone release.