In most mammals the gonads are under the control of the pituitary gonadotropins LH and FSH. However, in the common marmoset monkey Callithrix jacchus no LH is detectable in the pituitary but chorionic gonadotropin (CG) instead, normally produced in the placenta. This study investigated the mechanism of CG subunit activation in the pituitary and why humans do not express CG in the pituitary. 5'-RACE, EMSA and promoter-driven luciferase assays performed with the gonadotropic LT2 cells showed that marmoset monkey CG is GnRH-responsive and activated similar to human LH by the transcription factors SF1, Egr1 and Pitx1, and displayed a transcriptional start site (TSS) 7 bp upstream of exon 1. In contrast, the human CG promoter displayed in the binding elements for Pitx1 and Egr1 three consensus sequence mismatches, leading to very low activity that could be drastically increased by mutation to the consensus sequences. Vice versa, marmoset CG promoter activity was reduced after introduction of the human CG mismatches. An in vivo study in pregnant marmoset monkeys showed that during pregnancy there is no significant decrease of pituitary CG production, contrasting human LH down-regulation. In conclusion, pituitary CG production is lacking in humans due to the absence of appropriate DNA-binding elements, which are present in marmosets, thereby enabling GnRH-activation of expression. However, during pregnancy of marmosets, pituitary CG expression is not inhibited.