Regulated expression of the erythropoietin (EPO) gene in the adult kidney plays a key role ill the regulation of erythropoiesis. However, uncertainty exists regarding the type of kidney cells involved in EPO gene expression, We previously showed by light microscopy that the lacZ reporter gene is expressed and inducible by hypoxia/anemia in the proximal convoluted tubular (PCT) cells of the kidneys of transgenic mice carrying the 5'-lacZ construct, in which the lacZ gene was placed downstream of a 7.0-kb mouse EPO gene segment containing 6.5 kb of the 5'-flanking sequence, We, report here the light and transmission electron microscopic examination of lacZ expression in the kidneys of transgenic mice carrying the 5'-lacZ construct and two additional constructs carrying the 6.5-kb 5'-flanking sequence with the body of the gene alone, or along with the 1.2-kb 3'-flanking sequence. The electron microscopic analyses unequivocally demonstrated that lacZ under the regulatory control of the 6.5-kb 5'-flanking sequence with or without the body of the gene and the 1.2-kb 3'-flanking sequence was expressed predominantly in the proximal convoluted tubular cells of the kidney following hypoxia induction. (C) 1997 Academic Press.