The peripheral nervous system and the immune system were shown to have neurohumoral interactions. This study extends observations that demonstrated neuronal modulation of spontaneous interleukin-6 (IL-6) secretion in the spleen by norepinephrine (NE) and -endorphin. Spontaneous IL-6 secretion in vivo was markedly reduced by removal of macrophages with the clodronate technique. Furthermore, spontaneous IL-6 secretion was significantly inhibited at physiological concentrations of cortisol (10⁷ M). In the presence of 10⁷ M cortisol, addition of norepinephrine (NE; 10⁵ M) and isoproterenol (10⁶ and 10⁵ M) significantly increased spontaneous IL-6 secretion (+20%; P = 0.0280, P = 0.0005, and P = 0.0050, respectively). In contrast, addition of -endorphin significantly inhibited spontaneous IL-6 secretion in the presence of 10⁷ M cortisol (40%; 10¹¹ M, P = 0.0410; 10¹⁰ M, P = 0.0005). To study the effect of endogenously released transmitters on spontaneous IL-6 secretion, spleen slices were electrically stimulated with 1, 5, 10, 50, and 100 Hz. Spontaneous IL-6 secretion was markedly reduced at a frequency of 10 Hz with 10⁷ M cortisol present (P < 0.0001). This indicates that the combination of nerve firing at 5-10 Hz and physiological cortisol conditions inhibits spontaneous IL-6 secretion. Inhibition of spontaneous IL-6 secretion from spleen macrophages is most probably due to a net inhibitory effect of opioidergic transmission under these conditions.