Timolol of 0.5% was applied in 6 eyes prior to cataract extraction, while 7 eyes were not treated. Aqueous humor was drawn after retrobulbar analgesia with 2% lidocaine and analyzed for epinephrine and norepinephrine using a radioenzymatic assay. Epinephrine levels averaged 15.8±16.2 and 19.3±27.5 pg/ml in the timolol treated group and controls respectively. The corresponding values for norepinephrine were 1.59±0.55 and 1.87±0.90 ng/ml. These differences were not statistically significant.Norepinephrine levels in this study were somewhat higher than in previous reports of catecholamine levels in human aqueous humor and considerably higher than plasma levels of norepinephrine. There was no significant correlation between aqueous humor norepinephrine levels and age in the two groups combined. The mean norepinephrine levels for men and women in the two groups combined were 1.73±0.89 and 1.76±0.56 ng/ml. The difference was not statistically significant. Timolol resulted in a decrease in the intraocular pressure.