In rats with chronic urinary tract infections, urine and blood were examined for two classes of compounds (nitrosamines and interferon) which may lead to the development of urothelial hyperplasia and neoplasia. In vitro, Escherichia coli, a Proteus species or a mixture of both were able to induce high levels of interferon which theoretically could reduce the host's cellular immune surveillance. These high levels were not detected in vivo in either short-term (5 hr. to 2 wk.) or long-term (2 wk. to 24 wk.) infected rats. In contrast, N, N dimethylnitrosamine was detected in the majority (greater than or equal to 50 per cent) of long term infected rats after 12 wk. although individual rats showed detectable levels as early as 2 wk. post infection. Sterile human or rat urine supported bacterial growth and subsequent production of N, N dimethylnitrosamine, but only after 16 wk. of subculturing in vitro. Gas chromatography was able to detect small amounts of nitrosamines extracted from urine. The mass spectrometer yielded quantitatively and qualitatively better detection. With long term infections, the appearance of a potential carcinogen, N, N dimethylnitrosamine, occurs in vivo and in vitro and correlates with previous findings that describe the development of hyperplastic and early neoplastic changes in the rat urothelium.