To examine whether the neuroendocrine hormone norepinephrine may influence the production of the Shiga-like toxins (SLTs), several Escherichia coli O157:H7 clinical isolates were grown in the presence or absence of norepinephrine. An in vitro culture system consisting of low (<1500 colony-forming units/ml) initial concentrations of inocula into a serum-based medium was used to more closely approximate in vivo conditions. The growth of all isolates was increased several logs in the presence of norepinephrine, as compared with the growth in controls, during a 24-hour growth period. Controls included additional dextrose as well as the use of the norepinephrine metabolite normetanephrine, which contains one more methyl group than norepinephrine and hence would serve as a better energy source for growth if the effect were solely nutritionally mediated. During the 24 hours of growth, the production of cell-associated SLT-I on a protein-equivalent basis was shown to be increased over 100-fold in norepinephrine-cultured bacteria as compared with controls. SLT-II elaboration into culture supernatants was also greatly increased in norepinephrine-cultured bacteria as compared with controls. Maximal detection of cell-associated SLT-II occurred at least 12 hours before maximum levels were achieved in culture supernatants. Because norepinephrine represents one of the largest pools of monoamines present throughout the small intestine, these results suggest that the neuroendocrine environment of the small intestine may play a role in the growth of O157:H7 and the production of SLTs.