A technique for measuring the increase in conjunctival thickness which accompanies a clinically relevant, topically induced model of ocular anaphylaxis was developed and was compared with the determination of an increase in percent of degranulated mast cells in affected tissue. Twenty rats immunized with egg albumin were topically challenged with egg albumin solution applied to one eye and with phosphate-buffered saline applied to the other eye. Clinical signs of edema and erythema were not seen in ocular tissues undergoing anaphylaxis. However, both conjunctival thickness and percent of degranulated mast cells were significantly greater in antigen-challenged tissues than in buffer-challenged tissues. Amount of edema correlated significantly with amount of mast cell degranulation. Conjunctival thickness served as a reliable indicator of anaphylaxis. The measurement of topically induced anaphylaxis by evaluating the increase in conjunctival thickness was simpler, faster, and less expensive than the evaluation of anaphylaxis by the increase in the percent of degranulated mast cells in the same tissues.