Previous in vivo studies demonstrated that estrogen treatment of male rats allows somatostatin (SRIF) to inhibit PRL release. The objective of this study was to determine whether chronic estrogen (E2) treatment of male rats can induce the conversion of somatotropes to mammosomatotropes. In situ hybridization and reverse hemolytic plaque assay were used to evaluate the effects of E2 treatment on GH and PRL messenger RNA (mRNA) content and hormone secretion in individual pituitary cells. Male rats were implanted for 2-6 weeks with placebo or estradiol-containing pellets (5mg/90-day release). Pituitaries were removed and prepared for reverse haemolytic plaque assay to determine PRL and GH secretion. This was followed by in situ hybridization using 35S-labeled riboprobes for PRL and GH mRNA. Chronic E2 treatment increased both the percentage of pituitary cells that secreted PRL and the amount of PRL secreted per cell. Concomitantly, there was a decrease in both the percentage of GH-secreting cells and that amount of GH secreted per cell. In situ hybridization demonstrated that E2 treatment increased PRL mRNA while decreasing GH mRNA in single pituitary cells. Significantly, in control male rat pituitary cell cultures, no PRL-secreting cells were positive for GH mRNA. In contrast, after chronic E2 treatment, 10% of PRL-secreting cells contained GH mRNA. In the control pituitary cell cultures, SRIF had no effect on PRL release, but SRIF significantly inhibited PRL release from pituitary cell cultures prepared from E2-treated male rats. These studies demonstrate that the adult pituitary preserves plasticity and, under the appropriate steroid milieu, allows conversion of somatotropes to mammosomatotropes.