A total of 104 eyes undergoing intraocular surgery were studied to investigate the effect on intraocular pressure (IOP) of peribulbar and retrobulbar anaesthesia in eyes with and without glaucoma. Forty eyes had glaucoma. Intraocular pressure was measured before, immediately after, and 5 minutes after injection of local anaesthetic. Mean IOP rose by 5.8 mm Hg at 1 minute (p < 0.01) and 0.7 mm Hg at 5 minutes (p > 0.05). However, in eyes not receiving external ocular compression after the 1 minute measurement (n = 70, 67%), IOP was still 3.6 mm Hg higher than baseline (p < 0.01), compared with 5.2 mm Hg lower than baseline (p < 0.01) where compression was used. Patients with glaucoma experienced higher and more persistent increases in IOP than those without glaucoma. The increase in IOP varied greatly between patients: the maximum rise was 25 mm Hg, and in one glaucoma patient an IOP of 50 mm Hg occurred, persisting for 5 minutes. At 1 minute, 14 of the glaucoma subjects (35%) had experienced an IOP rise of > or = 10 mm Hg, and four (10%) a rise of > or = 20 mm Hg. These results suggest that the changes in IOP in patients with glaucoma, with an acute increase in IOP being succeeded by an acute decrease on entry into the anterior chamber, may be hazardous. The implications for clinical practice are discussed.