The 11b-Hydroxysteroid dehydrogenases (11bHSD) are a subfamily of short-chain dehydrogenase/reductase enzymes, which regulate cortisol metabolism and actions in vivo. Although much is known about the biology and function of 11bHSD1 and 11bHSD2, little is known of a recently discovered member of the 11bHSD family, called 11bHSD1L which is expressed in primates, fish, plants, and other mammals, but is absent in rodents (1). Current expression profiling in human, sheep and marmoset tissues suggest that 11bHSD1L may be linked to the regulation of reproduction through the hypothalamic-pituitary-gonadal (HPG) axis or to an unknown regulatory role in the brain. The definitive physiological substrate for 11bHSD1L is unknown. We have studied the expression of 11bHSD1L in the non-human primate marmoset and show that 11bHSD1L mRNA levels were highest in regions of the brain and reproductive organs, which parallels expression patterns in human tissues. Analysis of two major 11bHSD1L RNA splice forms, derived from exon 8, identified that the 11bHSD1L ‘Exon 8A’ splice variant was the dominant isoform with mRNA levels 100 times that of the ‘Exon 8B’ isoform in the marmoset ovary and cortex of the brain. Localization of 11bHSD1L protein in marmoset tissue sections by immunohistochemistry identified several cell populations previously unreported for 11bHSD1L, that included Sertoli cells in the testis, and theca cells in the ovary - further implicating 11bHSD1L as a potential novel regulator of the HPG Axis. There is some evidence that 11bHSD1L is a weak oxidase of the steroid cortisol. Preliminary experiments using a dual-luciferase based cell transfection reporter system shows that cortisol is unlikely to be a substrate of 11bHSD1L, distinguishing it from other members of the 11bHSD subfamily of enzymes.