We attempted to determine whether the sympathetic nervous system in rodents is susceptible to co-carcinogenesis, employing murine salivary nerve growth factor (NGF) as a co-carcinogenic agent. NGF had no co-carcinogenic effect with either methylcholanthrene or ethylnitrosourea (ENU) on the sympathetic nervous system of the mouse, whether administered transplacentally, postnatally, or both transplacentally and postnatally. At a dose of ENU of 30 mug/g body weight, NGF did not shorten the latent period for tumor induction of BD-IX rats. In contrast, a 25% reduction in latent period was brough about by NGF for tumor appearance in BD-IX rats receiving 90 mug/g ENU. In both cases the frequency of urogenital tumors in rats was increased as a result of NGF administration, at the apparent expense of neural tumors.