The lactotroph undergoes dynamic regulation of cell cycle progression during pregnancy, as well as throughout the de- velopment of the pituitary. We recently reported that female mice with targeted disruption of Cdk4, one of the G1-regula- tory cyclin-dependent kinases, are unable to support embryo implantation because of defective progesterone secretion from the corpus luteum. In this study, we demonstrate that this phenotype is not attributable to a primary defect in the corpus luteum but is a consequence of defective prolactin (PRL) production caused by inappropriate development of the pituitary lactotroph population. Specifically, the pitu- itary of Cdk4-deficient mice is extremely hypoplastic. Lac- totrophs and somatotrophs of prepubertal Cdk4-deficient mice were 80% decreased in number, relative to those in wild- type mice, whereas gonadotrophs were unaffected. Lac- totrophs of Cdk4-deficient mice did not proliferate in re- sponse to estrogen administration, whereas estrogen could induce the expression of galanin, an estrogen-responsive fac- tor required for lactotroph proliferation. The reduction in lactotroph numbers was reflected by markedly diminished serum PRL levels in both prepubertal and postcoital Cdk4- deficient mice. Administration of PRL, after mating, signifi- cantly increased serum progesterone levels and restored im- plantation in Cdk4-deficient female mice. These observations demonstrate that Cdk4 is required for normal proliferation of the lactotroph population. (Endocrinology 143: 3001-3008, 2002)