Ten female rats, of the age of three months, were treated for 24 days with increasing doses (10 mg/kg daily in the first week, 20 mg/kg in the second week, 30 mg/kg in the third week and 40 mg/kg in the last three days of treatment) of morphine acetate injected intra peritoneum, in order to cause a chronic intoxication (13). Five rats were utilized as controls. In all cases the vaginal smears was daily examined in order to evaluate the possible changes induced by the drug administration. The results obtained show that morphine produces important alterations in the estrous cycle of all the treated animals. In fact from the second day of treatment all animals had the cycle blocked in metaestrous. Afterwords a great variability occurred with a complete lack of estrous for the whole experimental period in five rats and very irregular succession and length of the phases into other ones. After the interruption of the treatment a resumption of the normal cycle was observed.