By researches carried out in adult female rats we followed the effect of chronic administration of malathion on the serotonin level from hypothalamus, rhynencephalon, mesencephalon and cerebral cortex and on the hypophysio-ovarian system. The obtained results showed that malathion produces: 1) the increase of cerebral serotonin level through the active phases of the gonadal cycle (proestrus + oestrus); 2) the increase of the percent incidence of the gonadal active phases; 3) the decrease of the LH synthesis through the oestral cycle; 4) the increase of the FSH release during the active phases and the decrease of the LH release in the inactive phases of the ovarian cycle. The data show that malathion influences the reproductive function both by its cellular toxic action and by affecting the encephalic regulatory serotoninergic, besides acetylcholinergic, mechanism of the reproductive functions.