The cellular-molecular mechanisms of changes in immune system function were studied, including the production of lymphocyte activating factor (LAF) and interleukin-1 (IL-1). the effects of IL-1 on target cells, and subsequent cytokine signal transduction via the sphingomyelin pathway, in different types of stress. These experiments showed that stress of different durations and intensities induced the formation of lymphocyte activating factors by peritoneal macrophages and increased IL-la levels in mouse blood. but led to different changes in the responses of target thymocytes to the committing actions of IL-1beta. which correlated with changes in the level of the humoral immune response. These data coincided with differently-directed stress-induced changes in the activity of membrane-bound neutral sphingomyelinase. the key enzyme in the sphingomyelin cascade, in cerebral cortex membrane fraction P2 from mouse brains. The results obtained here suggest that IL-1 is involved in the physiological mechanisms of stress reactions, operating at the levels of IL-1 production and its biological actions on lymphoid target cells, as well as at the level of cytokine signal transduction via the sphingomyelin pathway in nerve tissue.