Beagles were treated with bisoprolol, a beta 1-selective adrenoceptor antagonist, for 30 days with the following daily doses: oral: 30 mg/kg; conjunctival: 0.5% solution (approx. 0.04 mg/kg) and 5% solution (approx. 0.4 mg/kg). Drug concentrations were determined in plasma and various eye tissues on days 1, 16, and 30, and on day 59, i.e. on day 29 of the follow-up period. Bisoprolol concentrations in plasma and most eye tissues were considerably higher after oral than after conjunctival treatment. The highest tissue concentrations were observed in the iris (+ciliary body) and retina (+choroid) with tissue/plasma concentration ratios between 100 and 150 after oral and 1000 to 3000 after conjunctival instillation (5% solution). In plasma no accumulation of the drug was observed which is in accordance with its plasma half-life of 4 to 5 h. In contrast to this, concentrations in the iris and retina increased from day 1 to day 16 and 30 by 3 to 8 times and the half-life of bisoprolol in these tissues was estimated to be between 3 to 5 days.