Abstract The effect of acute immunization or continuous environmental antigenic exposure on nor-adrenaline (NA) content of rat lymphoid organs was studied. Immunization with sheep red blood cells resulted in a decrease in splenic NA content. In other experiments, NA levels were determined in lymphoid and nonlymphoid organs of SPF rats exposed to naturally occurring environmental antigens, as well as in germ-free animals of the same strain. Spleen, thymus, and lymph nodes in SPF rats contained about 50% less NA than germ-free animals. With the exception of the adrenals, nonlymphoid organs showed no such a difference. Taken as a whole, the present results and our previous data on immunosuppressive influences of the sympathetic innervation on lymphoid organs support the notion that the sympathetic system plays a role in immunoregulation.