Inhibitory effects of the anti-inflammatory agents lysine-acetylsalicylate (LAS) and phenylbutazone (PBZ) on the breakdown of the blood-aqueous barrier by paracentesis were studied in canine eyes using protein determination of ocular fluid. In the untreated eyes the aqueous protein value was raised from 0.29 +/- 0.17 (mean +/- SD) g litre-1 at the initial paracentesis to 14.47 +/- 4.10 g litre-1 at the second paracentesis. Pretreatment with LAS or PBZ had no significant effect on the protein concentration of the primary aqueous humour. However the secondary aqueous protein concentration was only 10.05 +/- 7.00 g litre-1 with LAS and 5.80 +/- 3.83 g litre-1 with PBZ. With both drugs the maximum inhibitory effect was observed on the gammaglobulins and albumin. These results suggest that prostaglandins may be involved in the response of the canine eye to paracentesis and that premedication with LAS or PBZ may be of value in reducing postoperative ocular inflammation.