BACKGROUND AND AIM Local modulation of myocardial function by gene transfer or cell depositions constitutes a potential method of cardiac treatment. This study tested the morphology of myocardial plasmid gene transfer by catheter-based transendocardial injection (NOGA). METHODS Left ventricular morphology and electrical and mechanical characteristics were mapped in three dimensions. In two pigs, 0.10 mL of toluidine blue was injected at ten sites. In seven pigs, seven to ten injections of 0.10 mL saline containing 0.10 mg pCMV-LacZ expressing the enzyme beta-galactosidase and 0.10 mg phVEGF-A165 were given. The pigs were sacrificed after 3 days and gene expression was determined. RESULTS Macroscopically on the endocardial surface, all identified spots were located in the target area. However, along the transmyocardial axis, injections with color and plasmid were located randomly throughout the left ventricular wall from the endocardium to the epicardium. In each detected spot, gene expression of beta-galactosidase was observed in an approximate myocardial volume of 5 x 5 x 5 mm. Microscopically, the transfected cells were located typically at the tip of the injection scar. As a rule, 10 to 20 transfected cells were located at the end of the injection scar. In sections where expression of both transcripts was observed, 42% of the cells expressed both beta-galactosidase and vascular endothelial growth factors (VEGF), 32% only beta-galactosidase, and 26% only VEGF. CONCLUSIONS Myocardial gene transfer following magnetic guidance can be located precisely on the left ventricular inner surface. Within the myocardium, gene expression is local around the distal tip of the injection scar and is located randomly at every level of depth of the left ventricular wall.