In normal urothelium, superficial umbrella cells express four major integral membrane proteins, uroplakins UPIa, UPIb, UPII, and UPIIIa, which compose urothelial plaques. In the apical plasma membrane, urothelial plaques form microridges. During neoplastic changes, microridges are replaced by microvilli, while uroplakin expression is retained. We correlated individual uroplakin expression with apical plasma membrane structure, cytokeratin 20 expression, and urothelial cell proliferation (Ki-67). Male Wistar rats were treated with 0.05% N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) in drinking water, which caused flat hyperplasia with mild dysplasia, low-grade papillary urothelial carcinoma, invasive low- and high-grade papillary urothelial carcinoma and invasive squamous cell carcinoma with extensive keratinization, grade 2. During urothelial carcinogenesis, UPII expression was the most decreased in all urothelial lesions, while UPIa, UPIb, and UPIIIa expression was differently altered in different types of lesions. Superficial cells were covered with microvilli and ropy ridges, while microridges were disappearing. The expression of cytokeratin 20 was decreased and limited to superficial urothelial cells. Proliferation indices were increased, except for invasive squamous cell carcinoma with extensive keratinization. Our results indicate that during urothelial carcinogenesis the expression of UPII is diminished, suggesting that UPIb/UPIIIa heterodimer can still be formed, while heterodimer UPIa/UPII formation is disrupted. Correlation between decreased level of UPII expression and changed apical plasma membrane structure suggests that diminished expression of UPII hinders the urothelial plaque formation.