Most strains of mice injected intracerebrally with lymphocytic choriomeningitis virus grow to adulthood maintaining a persistent virus infection associated with chronic virus-induced immune complex disease. However, mice on a k background are highly susceptible to neonatal infection and develop the clinical syndrome of pituitary dwarfism and hypoglycemia. Examination of pituitary tissue fails to reveal morphologic alteration by light and electron microscopy. Within the pituitary, viral antigens are exclusively distributed within the cells of the adenohypophysis. Using ultrastructural colloidal gold-labeling techniques, we demonstrate the presence of mature virus particles budding from the surface of growth hormone containing cells from the pituitary. This study indicates that persistent lymphocytic choriomeningitis virus infection of the growth hormone cells in susceptible mice is associated with pituitary dwarfism without producing visible structural damage.