Laser irradiation of the iris in rabbits causes miosis, a rise in intraocular pressure and an increase in the protein content of the aqueous humour. These effects on intraocular pressure and the blood-aqueous barrier are partly due to the release of prostaglandins of the E type and can be very much reduced by pretreatment with prostaglandin synthetase inhibitors such as indomethacin. The miosis and part of the rise in intraocular pressure and breakdown of the blood-aqueous barrier may be due to antidromic stimulation of sensory nerves, and they are reduced by nerve-blocking drugs. A combination of indomethacin and local anaesthetic can block the miosis and rise in intraocular pressure and greatly reduce the increase in protein content of the aqueous humour. It is probable that the human eye reacts in a similar way to laser irradiation of the iris, but so far the rise in intraocular pressure has not had serious results clinically in the treatment of glaucomatous eyes.