This study investigates the effects of noradrenaline injection on a range of immunological activities in Sydney rock oysters (Saccostrea glomerata). Noradrenaline caused a decrease in most of the immunological parameters tested. Phenoloxidase activities in both whole hemolymph and serum decreased significantly within the first 60 min of noradrenaline injection, as did the total frequency of hemocytes in hemolymph, differential hemocyte frequencies, the frequency of phenoloxidase positive cells and phagocytic activity. All of these parameters started to return to normal levels within 120 min. In contrast, the total protein content of hemolymph, which also decreased after noradrenaline injection continued to decline throughout the experimental period. In vitro studies found that superoxide and peroxide production by hemocytes increased in the presence of noradrenaline, but acid phosphatase activity decreased significantly. Additional experiments showed that noradrenaline secretion was stimulated by altered salinity, altered temperature and physical agitation. This suggests that stressors commonly associated with oyster farming may result in noradrenaline-based stress responses that suppress immunological activity.