Medicament contact dermatitis can be troublesome in external ear disease. In this work the effects on the guinea pig external ear of instillation twice daily for 3 weeks of benzalkonium chloride, Cerumol, gentamicin, the pharmaceutical base for Gentisone HC, hydrocortisone and propylene glycol were studied using light microscopy, thickness measurements of tympanic membrane and meatal epidermis, and the mitotic index of meatal epidermis. Benzalkonium chloride induced severe inflammatory changes and markedly increased the thickness measurements and meatal mitotic index. Cerumol also increased the thickness measurements and the mitotic index; there was additionally low grade chronic inflammation in the dermis. Prophylene glycol increased the mitotic index and thickness measurements though no signs of inflammation were seen. Other compounds did not produce significant changes except that hydrocortisone reduced the mitotic index. Contact dermatitis to benzalkonium chloride may be important in persistent ear disease. Cerumenolytic agents ought not to be used for prolonged periods or as prophylaxis.