1. The authors report the results obtained after the action of certain optotoxic substances on several enzyme activities in the retina of the pig. 2. This in vitro study involved enzyme interferences of the following optotoxic agents : ethionamide, d-penicillamine, ethylene diaminotetra-acetic acid (EDTA), disodium and dicobalt salts. The enzyme activities studied involved glycolysis, the enzymes selected being as follows: glucose phosphate isomerase (GPI, E.C. 5.3.1.9), fructose-1,6-diphosphate aldolase (F1-6diPA, E.C. 4.1.2.13), lactate dehydrogenase (LDH, E.C. 1.1.1.27). 3. Following the action of the effectors studied, a marked decrease in the enzyme activities examined was found in the retina. This decrease, of varying rapidity and regularity, went as far in some cases as total inhibition; there was disturbance of glycolysis. 4. These results indicate the existence of interactions with a complex mechanism. It may be noted that all of the effectors studied were chelating agents of divalent metals and the changes which they induced in the enzyme activities examined may be explained by interference of the chelates formed with metal cations, such as Zn++, co-factors or effectors of these glycolysis enzymes (with the exception of GPI). These stable chelates are formed by virtue of the primary amine--NH2, thiol--SH, thionyl-[Formula: see text] groups, i.e. groups belonging to molecules essential to cell metabolism.