AbstractAdult female Sprague Dawley rats were divided into three groups and given two daily doses of: 1) morphine (25 mg/kg at each injection), or 2) methadone (5 mg/kg), or 3) saline for nine days. Two hours before the rats were killed on day 10, they were given a double dose of the drugs. Fifteen minutes before being killed, some of the morphine‐treated rats were given the opiate antagonist naloxone (2.5 mg/kg), which caused a sudden arousal in these rats. At the time of killing, the preoptic‐hypothalamic region was rapidly removed and frozen in liquid propane to prevent translocation of elements in cells. Frozen 4‐μm sections were cut, freeze‐dried, and electron‐probed in a scanning electron microscope, and energy‐dispersive x‐ray spectra were collected. The characteristic peak‐to‐continuum ratio for all detectable elements was determined in both the nucleus and the cytoplasm of ependymal cells and neurons. The data from nine cells of each type in each rat brain were then subjected to one‐ and three‐way analysis of variance. The results show significant differences in the distribution of elements (sodium, magnesium, phosphorus, sulfur, chlorine, potassium, and calcium) which are dependent upon: 1) subcellular localization, 2) cell type, and particularly, 3) opiate treatment. The behavioral state produced by the opiate is correlated with the effects they have on intracellular concentration of several elements