Implantation of 1 x 10(6) acutely dispersed rat pituitary cells into the lateral ventricles of sexually immature and mature female rats of both the Sprague-Dawley and Fisher strains significantly suppressed the weight and protein content of the in situ pituitary 12 days postimplantation. Body weight gains in the young animals were also significantly lower. Changes in the in situ pituitary of the cell implanted groups were demonstrated by repeatable reductions (approximately 50%) in total pituitary GH and PRL content. Total pituitary TSH, FSH and LH were usually unaffected. Although the mechanism of cell-mediated suppression of the host pituitary is unknown, the approach should be of use in future studies of feedback regulation on pituitary gland function.