Pathological studies were performed on the five piglets experimentally infected with Aujeszky's disease virus(pseudorabies), NYJ isolate, isolated from the naturally infected pigs in Korea: two piglets were inoculated intramuscularly, two piglets intranasally, and one piglet subcutaneously at the dose of 1$m\ell$ per animal with the 105.5 $TCID_50$/0.1ml titer. Clinical signs included dyspnea, high fever(＞$41^{\circ}C$), anorexia, vomiting, diarrhea or constipation, ataxia, circling movement, posterior paralysis, intermittent convulsion, and coma followed by death although some variations by age and inoculated routes were observed. Gross features included multiple necrotic foci in the liver, congestion and hemorrhage in the lymph nodes and spleen, petechial hemorrhage in the kidney, hemorrhagic pneumonia, marked meningeal congestion, severe sub meningeal hemorrhage in the spinal cord, excessive cerebrospinal fluid retention, and muscular necrosis at the inoculated area. Microscopically, non suppurative meningoencephalitis with gliosis and perivascular cuffing in CNS, ganglioneuritis, necrohemorrhagic splenitis, necrotic hepatitis, tonsillitis and rhinitis, hemorrhagic or interstitial pneumonia, and non-suppurative myositis in the injected area were observed. Eosinophilic intranuclear inclusion bodies were found in a variety of tissues the including the liver, kidney, adrenal gland, spleen, lymph nodes, tonsil, and lung. Ultrastructurally, virus particles were confirmed in nucleus and cytoplasms of pneumocytes around the necrotic areas.