The concentration of the metabolite of the beta-blocker metipranolol was determined in the aqueous humour of 89 cataract patients. At 1, 2 or 5 h before surgery, they received one drop (30 microliters) of a 0.1% or 0.3% solution of the drug. At 1, 2 or 5 h after the application of 0.1% metipranolol eye drops, desacetyl-metipranolol concentrations of 624.55, 235.29 and 88.02 ng/ml, respectively, were measured. At the same intervals after the instillation of 0.3% metipranolol eye drops, the respective values of 1289.20, 1120.88 and 327.36 ng/ml were found. The metabolite concentration in the eye drops and the values measured show no consistent correlation.