• Δ9-Tetrahydrocannabinol (THC) or vehicle alone was applied topically to one eye of normal paid volunteers. Ocular and systemic toxic effects and intraocular responses were measured in different series. Toxicity was limited to minor conjunctival injection that was of short (< 60 minutes) duration and occurred with both drug and vehicle alone. Subjective responses indicated a sensation of minor burning and/or tearing. No fall in intraocular pressure was found. A small (1 mm) but significant mydriasis occurred in both the treated eye and untreated eye and was not drug related. Single-drop administration of Δ9-THC did not, therefore, cause any significant ocular irritation or reduce IOP.