Rats were treated with either morphine or placebo by implantation of subcutaneous pellets. After one week, morphine-treated rats were addicted, as demonstrated by naloxone-induced abstinence. The animals were sacrificed by perfusion with buffered aldehyde fixative. The superior cervical ganglia (SCG), kidneys, and other tissues were removed and processed for light and electron microscopy. In the SCG, the percentage of secondary lysosomes, compared to primary lysosomes, was higher in the morphine-treated rats than in control rats (p Less than 0.05). There was a strong trend (0.1Greater Than p Greater than 0.05) for the range of mitchondrial diameters to increase, suggesting that some mitochondria were enlarged and others were condensed in morphine-treated rats. In the kidney, both short (p Less than 0.05) and long (p Less than 0.01) microvilli increased in frequency on the surface of glomerular podocytes following morphine treatment. These results strongly resemble those obtained in studies of the aging superior servical ganglion and kidney, and suggest that morphine can cause ultrastructural alterations which mimic changes normally occurring with age.