The authors have described the connections between nervous and immune systems. Endogenous opioids are one of the factors linking both systems. Endogenous and exogenous opioids can modify the function of interferons, humoral factors, antibody production and lymphocytes' activation. Concurrently, some cytokines can modify the endogenous opioid system. The aim of this study was to asses whether the subcutaneous administration of IL-2 influences the Met-Enkephalins concentration in serum after IL-2 single administration due to renal cancer. The Met-Enk level was estimated with RIA method. The IL-2 single administration results in a significant decrease of Met-Enk in serum.