The role of interleukin 4(IL-4) as an inflammatory mediator during the immune labyrinthitis was examined in the rat inner eat using in situ hybridization. Leukocytes presented in the scala tympani and scala vestibuli and the endothelial cells in the spiral modiolar venules after injection of keyhole limpet hemocyanin(KLH) into the scala tympani of immunized animals were labeled with the IL-4 probe. Inflammatory cells in the perisaccular connective tissue area of the endolymphatic sac were also labeled with the IL-4 probe. The results suggest that IL-4 may play a role of an immunoregulator and inflammatory mediator for the systemic and local immune reactions during the progress of inflammation in the inner ear.