Changes in immunoreactivity in rat pineal gland were investigated in S‐antigen (S‐Ag)‐induced pinealitis. Anti‐S antibodies were used to study the accessibility of pathogenic epitopes in the organ, and phosphatidyl inositol 4, 5‐bisphospate (PI[4,5]P2) antibodies were chosen to study changes in functional phospholipid in membranes. Control pineal gland (without inflammation) did not show much reactivity toward these antibodies. Immunoreactivity toward both antibodies was markedly increased throughout the organ at the peak of inflammation and decreased almost to control level in the postinflammatory stage. The rise and fall of immunoreactivity were attributed to changes in the accessibility of antigenic sites rather than changes in the amounts of S‐Ag and P(4, 5)P2The transient enhancement in immunoreactivity, probably consequential to tissue damage, suggests that the pineal gland, unlike the retinal photoreceptors, is capable of quick repai