The ocular hypertension caused by the μ agonist alfentanil (10 μg/kg, i.v.) in spontaneously breathing rabbits was reversed to ocular hypotension by pretreatment with the k agonist-μ antagonist, nalbuphine (0.6 mg/kg, i.v.). This is probably due to nalbuphine’s μ antagonistic action which prevents the respiratory-depressant effect of alfentanil that elevates the blood pCO<sub>2</sub> with a drop in blood pH. It is known that hypoxia and hypercarbia lead to uveal vasodilation and increase in aqueous production which cause elevation of intraocular pressure. Both alfentanil and nalbuphine have miotic effects when used individually. However, no further reduction in the pupil size was noted by their combined use. These results indicate that using nalbuphine and alfentanil together may be considered a safe combination for eye surgery because it preserves the analgesic effect of either drug while prevents the undesirable respiratory-center depression and the ocular hypertension observed with alfentanil alone.