In earlier experiments endorphin treatment of newborn rats caused the decrease of brain serotonin content, increasing aggressivity, enhanced sexual activity of females and changes in the binding capacity of uterine estrogen receptors at adult age, however nociceptin content of the cerebrospinal fluid was not changed. In the present experiment neonatal treatment of male and female rats was done with a single dose of 3 microg beta-endorphin and in five months old rats the level of nociceptin antagonist nocistatin was determined by radioimmunoassay in the cerebrospinal fluid. In both genders the amount of nocistatin was one magnitude higher in the endorphin treated groups. There was also a significant difference between the male and female nocistatin level in the treated and non-treated groups alike, with the advantage of females. The results call attention to the possibility of influencing pain-tolerance for life, by the pain-provoked endorphin levels during delivery.