Aberrant wnt-signaling caused by mutations in CTNNB1 occurs in about 15% of Wilms tumors. Nuclear beta-catenin protein, a substitute marker of active wnt-signaling, has been detected in an even higher proportion (>50%) of Wilms tumors suggesting alternative genetic pathways leading to beta-catenin activation. Thus, targeting wnt-signaling may become an important future therapeutic strategy in Wilms tumor patients. Currently, chemically-induced rat nephroblastomas provide the only available rodent model for this tumor. To determine the contribution of active wnt-signaling in this model, we investigated 25 chemically-induced rat nephroblastomas for beta-catenin protein expression and for Ctnnb1 exon 3 mutations. Using immunohistochemistry 16 of 25 tumors showed strong nuclear accumulation of beta-catenin protein although in a heterogenous pattern. Blastemal and mesenchymal compartments displayed nuclear positive cells more frequently than areas of epithelial differentiation. Interestingly, we found no mutation of exon 3 of Ctnnb1 in any of the 25 tumors analysed. In conclusion, our findings suggest activation of wnt-signaling in the vast majority of chemically-induced rat nephroblastomas. Nuclear expression of beta-catenin in the absence of Ctnnb1 mutations implies, however, alternate mutational targets in rat nephroblastomas. Chemically-induced rat nephroblastomas may constitute a suitable model system to test future anti-cancer drugs targeting the wnt-signaling pathway. betabeta.