The intraocular penetration of the aminoglycoside netilmicin following intramuscular and subconjunctival injection was studied in 102 patients undergoing elective cataract surgery. Those who received either a single 1.5-mg/kg intramuscular injection or two 1.5-mg/kg injections 6 hours apart subsequently had therapeutic serum levels but aqueous levels of less than 2.4 micrograms/mL. Those who received a subconjunctival injection of either 12.5 or 25 mg of netilmicin subsequently had aqueous levels of up to 36 or 85.6 micrograms/mL; therapeutic levels in the anterior chamber persisted for 7 to 9 hours, with higher levels following the 25-mg injection. Despite wide variation in the aqueous levels following subconjunctival injection the ocular penetration of netilmicin was comparable to that reported for other aminoglycosides. No major complications were associated with the use of this antibiotic by either route.