The human (h) growth hormone/chorionic somatomammotropin (GH/CS) gene locus presents a unique model to gain insight into the molecular mechanisms that have allowed a closely related family of genes to be expressed in two distinct cell lineages/tissues: pituitary somatotrophs and placental syncytiotrophoblasts. However, studies of external factors that regulate gene expression have been somewhat limited by (i) a lack of human cell lines expressing endogenous GH or CS appropriately; and (ii) the fact that the GH/CS locus is unique to primates and thus does not exist in rodents. In the current study, a transgenic (171 h GH/CS-TG) mouse was generated containing the intact hGH/CS gene cluster and hGH locus control region (LCR) in a 171-kilobase DNA fragment. Pituitary and placental-specific expression of hGH/CS RNA was detected at embryonic day (E) 18.5. Immunostaining of hGH was seen in somatotrophs of the anterior pituitary beginning in late gestation. The presence of hCS protein was detected in the placental labyrinth in trophoblasts functionally analogous to the syncytiotrophoblast of the chorionic villi. This pattern of gene expression is consistent with the presence of essential components of the hGH/CS LCR. Transcript levels for hCS-A, hCS-B and placental hGH-variant increased in 171 hGH/CS-TG placenta during gestation (E11.5-E18.5), as previously observed in human placental development. Throughout gestation, hCS-A RNA levels were proportionately higher, accounting for 91% of total CS RNA by E18.5, comparable to term human placenta. Finally, the previous correlation between the transcription factor AP-2alpha and hCS RNA expression observed in developing primary human cytotrophoblast cultures, was extended to pregnancy in the 171 hGH/CS-TG mouse. The 171 hGH/CS-TG mouse thus provides a model to investigate hGH/CS gene expression, including in pregnancy.