We have previously reported that transgenic mice containing the human renin gene express high levels of human renin mRNA in the lung. We show in this report that human renin expression in two lines of transgenic mice is developmentally regulated. Human renin expression is not evident in the transgenic mouse lung at 15.5 days of gestation, is detectable at 17.5 days of gestation, peaks around birth, and remains elevated into adulthood. In situ hybridization of mouse fetal lung samples at 18.5 days of gestation revealed that human renin was exclusively expressed in pulmonary type II epithelial cells. A survey of the medical literature revealed a number of clinical cases in which hypertension was caused by renin-secreting pulmonary tumors and a fairly widespread occurrence of immunoreactive renin in banked pulmonary tumors of diverse origin. This prompted us to examine a number of pulmonary tumor cell lines to determine whether they express human renin mRNA. One pulmonary carcinoma cell line, CALU-6, expressed human renin mRNA endogenously. Human renin expression in these cells was induced approximately 100-fold after treatment with forskolin, 8-bromoadenosine 3′:5′-cyclic monophosphate, or N⁶,2′-O-dibutyryladenosine 3′:5′-cyclic monophosphate. Transfection analysis of human renin promoter–luciferase fusion constructs revealed the presence of cell-specific positive and negative regulatory elements in the human renin 5′-flanking DNA. This cell line is the only immortalized human cell line that expresses high levels of endogenous human renin mRNA and should provide an excellent tool for studying the regulation of human renin expression in vitro.