Although cholecystokinin is a regulatory peptide with a predominant role in the brain and the gastrointestinal tract, there is an increasing evidence for its role in the kidney. The aim of this study was to reveal morphological changes in the structure of kidney of mice with cholecystokinin overexpression by means of light, transmission and scanning electron microscope, and atomic force microscopy. Using immunohistochemistry the expression of important basement membrane proteins collagen IV, laminin and fibronectin, as well the distribution of cholecystokinin-8 in the renal structures was evaluated. The altered morphology of kidneys of mice with cholecystokinin overexpression was seen by all microscopic techniques used. The renal corpuscles were relatively small with narrow capsular lumen. The basement membranes of renal tubules were thickened and the epithelial cells were damaged, which was more pronounced for distal tubules. Characteristic feature was the increased number of vesicles seen throughout the epithelial cells of proximal and especially in distal tubules reflecting to the enhanced cellular degeneration. The relative expression of laminin but not collagen IV in the glomerular basement membrane was higher than in the tubular basement membranes. The content of fibronectin, in opposite, was higher in tubular membranes. Cholecystokinin-8 was clearly expressed in the glomeruli, in Bowman's capsule, in proximal and distal tubules, and in collecting ducts. Ultrastructural studies showed irregularly thickened glomerular basement membranes to which elongated cytopodia of differently shaped podocytes were attached. As foot processes were often fused the number of filtration pores was decreased. In conclusion, cholecystokinin plays important role in renal structural formation and in functioning as different aspects of urine production in mice with cholecystokinin overexpression are affected-the uneven glomerular basement membrane thickening, structural changes in podocytes and in filtration slits affect glomerular filtration, while damaged tubular epithelial cells and changed composition of thickened tubular basement membranes affect reabsorption.