Cannabinol or cannabigerol was administered to cats topically in doses of 250, 500 and 1000 micrograms as a single drop or chronically via osmotic minipumps (20 micrograms hr-1) over a period of 9 days. While cannabinol had a modest effect on intraocular pressure after a single dose, it caused a more significant reduction in ocular tension during chronic administration. Cannabigerol had similar effects, but the magnitude of response to its chronic administration was greater. Cannabinol but not cannabigerol caused conjunctival erythema and hyperemia. After systemic administration of cannabinol (20, 40 or 80 mg kg-1) to rats, 8-13 Hz polyspike discharges appeared in the electrocorticogram during wakefulness and during rapid eye movement sleep episodes. Cannabigerol (10, 30 and 100 mg kg-1) lacked this effect. These results indicate that chronic administration of these cannabinoids lowers ocular tension considerably. Like marihuana and delta-9-tetrahydrocannabinol, cannabinol produced both ocular toxicity and neurotoxicity. As cannabigerol lacked these toxicities, it appears that the ocular hypotensive effect of this cannabinoid is somewhat dissociable from both the adverse central and ocular effects accompanying marihuana intake.