The effects of ammonia and urea on branchial carbonic anhydrase (CA) enzyme which plays a key role in ionoregulation, osmoregulation and acid-base balance of rainbow trout (Oncorhynchus mykiss) were investigated. CA activity of the control group for ammonia and urea was determined as 1285.7 +/- 67.9 and 1261.7 +/- 60.8 EU/mg protein, respectively. The CA enzyme activities of the other groups were measured at 1, 2 and 3 h after ammonia and urea applications. The corresponding activities of ammonia were 774.9 +/- 68.8, 732.1 +/- 48.6 and 768.1 +/- 59.5 EU/mg protein, respectively and that of urea were 769.3 +/- 58.9, 638.2 +/- 147.7 and 1108.1 +/- 61.1 EU/mg protein, respectively. The differences between the initial CA activities for the controls was not significantly (P > 0.01). The CA activities were significantly (P < 0.01) inhibited both in ammonia and urea group. However, the ammonia inhibited more than urea since there was significant differences between final values of gill CA activities. (C) 2004 Elsevier B.V.. All rights reserved.