Lymphocytes were separated from peripheral blood and incubated in Hanks’solution with calcium or buffer solution containing phosphodiesterase inhibitor IBMX. When carbachol at 1-10mmol/L was added to the incubation medium

the intracellular cGMP colltent as determined by high-sensitivity RIA was elevated by 66%- 80%. Such an elevation could be blocked by atropine and hence is a specific reaction mediated through M-receptors. A method based on this reaction was thus established to reflect the reactivity of M-receptor/cGMP system. Preliminary experiment revealed that the base level of lymphocyte cGMP as well as the elevation of cGMP induced by carbachol in aged（24-26 months old） rats were significantly lower than young rats. The methodology

especially the influence of incubation conditions was discussed.