Levocabastine is a very potent histamine H1 antagonist, available for ocular use as eye drops containing 0.5 mg/ml of levocabastine with benzalkonium chloride as a preservative. Local tolerability of the eye drops was evaluated by objective measurements including slit lamp, tonometry, fluorescein and rose Bengal staining, ophthalmoscopy, visual acuity and visual field determinations, as well as subjectively by questioning for adverse experiences. Seven ocular tolerability studies were performed in a total of 71 volunteers and 103 patients with allergic conjunctivitis. Levocabasine was given from twice to four times daily for 1-8 weeks. Two studies were open, three were placebo controlled and in two, levocabastine was compared with terfenadine. No changes of either clinical or statistical significance were observed in the ocular or peri-ocular tissues. Subjectively, local irritation, pain and tearing were virtually absent. Detailed ophthalmological tolerance studies therefore showed levocabastine eye drops to be very well tolerated.