To define the embryonic origin and lineage of the juxtaglomerular (JG) cell, transplantation of embryonic kidneys between genetically marked and wild-type mice; labeling studies for renin, smooth muscle, and endothelial cells at different developmental stages; and single cell RT-PCR for renin and other cell identity markers in prevascular kidneys were performed. From embryonic kidney day 12 to day 15 (E12 to E15), renin cells did not yet express smooth muscle or endothelial markers. At E16 renin cells acquired smooth muscle but not endothelial markers, indicating that these cells are not related to the endothelial lineage, and that the smooth muscle phenotype is a later event in the differentiation of the JG cell. Prevascular genetically labeled E12 mouse kidneys transplanted into the anterior chamber of the eye or under the kidney capsule of adult mice demonstrated that renin cell progenitors originating within the metanephric blastema differentiated in situ to JG cells. We conclude that JG cells originate from the metanephric mesenchyme rather than from an extrarenal source. We propose that renin cells are less differentiated than (and have the capability to give rise to) smooth muscle cells of the renal arterioles.