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The possible correlation between the severity of chronic progressive glomerulonephropathy (CPN) and the incidence of adrenal pheochromocytoma was examined in selected studies of male Fischer 344 (F344) rats at the National Toxicology Program (NTP). The NTP historical control database was first examined in order to determine whether there was association between the severity of CPN and the occurrence of adrenal pheochromocytoma in unexposed animals. Following this analysis, the 125 most recent NTP studies conducted in F344 rats were examined in order to determine how frequently chemicals that cause increased severity of CPN showed an increased incidence of pheochromocytoma. Finally, we examined the association between the incidence of pheochromocytoma and the severity of CPN in those NTP studies with chemically related increased rates of pheochromocytoma. In control male F344 rats surviving beyond 21 mo, the incidence of adrenal pheochromocytoma was consistently higher in animals with more severe CPN. This association was significant (p < 0.05) both for 900 NTP inhalation study controls and 900 NTP feeding study controls. An association was not consistently observed when dosed groups were considered. Although 22% (28/125) of NTP studies reported a chemically related increased severity of CPN, only 3 of these reported a corresponding significant increase in the incidence of pheochromocytoma. Of 6 NTP studies that reported increased incidence of pheochromocytoma, animals with pheochromocytoma from 5 of those studies had some degree of increased severity of CPN. However, the estimated strength of the correlation with the severity of CPN varied from study to study and was often quite different from that indicated by an analysis of the more extensive NTP control databases. The possible correlation between the severity of CPN and the incidence of pheochromocytoma may influence interpretation of carcinogenic effects observed at this site.