Systemic carbonic anhydrase (CA) inhibitors are among the most powerful agents to lower intraocular pressure. Unfortunately, their use is frequently accompanied by undesired side effects. Some are due to the relatively large amounts of drug that have to be systematically administered to inhibit the CA in the ciliary processes. The aim of the present work was to study in vitro effects of some pesticides on CA enzyme obtained from blood of fish, which play a key role in salt- and osmoregulation and acid-base balance in the fish, Oncorhynchus mykiss and Cyprinus carpio carpio living in freshwaters, and compared with CA inhibitors. CA activities were significantly inhibited by pesticides and inhibitors. I-50 values of O. mykiss CA enzyme inhibited by lambda-cyhalothrin, deltametrin, diozinon, dorzolamide and brinzolarnide were 6.05 x 10(-4), 1.48 x 10(-5), 6.84 x 10(-3), 3.82 x 10(-5) and 1.80 x 10(-6) molA, and that for C c. carpio 6.86 x 10(-4),4.70 x 10(-4), 3.92 x 10(-3), 8.34 x 10(-6) and 1.42 x 10(-6) molA, respectively. The pesticides used in this study inhibited the CA activity from different fish species to various degrees. It was found that the most effective inhibitor of CA enzyme within pesticides used was detrametrin. These findings observed in vitro could be useful in the understanding of the toxic effects that pesticides elicit on aquatic organisms in vivo. (c) 2005 Elsevier B.V. All rights reserved.