N-myc oncogene expression was characterized in porcine kidneys to investigate the potential role of this gene in normal renal development and oncogenesis. N-myc RNA expression was detected in porcine kidneys from birth until 5 wk of age, which corresponds to the time when glomerular differentiation is completed. Immunohistochemical studies revealed that N-myc protein was selectively expressed in the primordia of renal proximal tubule epithelial cells. These cells were cultured in vitro and continued to express N-myc for a limited time. Comicroinjection of a mutant ras oncogene and N-myc into these cells led to focus formation in soft agar, loss of contact inhibition, and the establishment of an immortalized cell line. These findings support a multistep model of renal oncogenesis that involves overexpression of N-myc.