Concentrations of metipranolol were determined in the aqueous humour of patients undergoing cataract surgery. At 30 min before the operation, patients were given 20- or 30 microliters drops of 0.1% or 0.3% metipranolol solutions with a viscosity of 7.5 cP or drops (20 or 30 microliters) of a 0.3% solution with a viscosity of 1.5 cP. Samples of the aqueous humour were obtained at the beginning of the cataract operation, and desacetyl-metipranolol content was determined by means of gas chromatography and mass spectroscope. The Wilcoxon signed-rank test and Jonckheere's method were used for evaluation of the results. A 3-fold increase in the strength of metipranolol was associated with an approx. 8-fold increase in the concentration of metabolite found in the aqueous humour. Drop size had no apparent effect on the concentration of this substance in the aqueous humour, but viscosity had a significant influence when large drops were applied. An increase in the amount of metipranolol applied as eye-drops was associated with an increase in the intraocular concentration of the metabolite.