The symmetrical triazine herbicides have been used for the preemergence control of broadleaf weeds for nearly three decades. Recently, certain members of this class, primarily the chlorotriazines (substituted in the 2 position), have been shown to evoke an increased incidence of mammary tumors in female Sprague‐Dawley rats. This response was noted when these chemicals were administered in the diet for 2 yr, and most often at dietary feeding levels at or above the maximum tolerated dose (MTD). At levels exceeding the MTD the health of these animals was compromised, as manifested by toxicity‐related reduced survival that was not associated with the occurrence of mammary tumors. Mammary tumors in rats frequently occur as a result of the influence of endogenous estradiol and prolactin. Those hormones, as well as progesterone, growth‐stimulating, luteinizing, and follicle‐stimulating hormones, were measured after 24 mo of dietary administration of the chlorotriazine, simazine. The plasma hormone pattern seen in aged female Sprague‐Dawley rats administered 1000 ppm simazine in the diet for 24 mo resembled that noted for aged female controls, except that the difference was more pronounced in the simazine‐treated group. These results suggest that prolonged exposure of Sprague‐Dawley females to excessive levels of triazines affects the neuroendocrine system, which in turn alters the pathology of the mammary gland. These changes are comparable to those that occur naturally as the rat ages. Changes in neuroendocrine control could result in the expression of an earlier onset and/or an increased incidence of mammary tumors, which already occur at a high spontaneous rate in aging Sprague‐Dawley female rats.