This study was carried out on 100 of healthy live common carp, (Cyprinus carpio) with an average body weight of 54.64+ 5.96 g and average total length 15.18 0.69cm. The fish were reared in aerated metalic tanks with flow rate of 8 L/h water. Fish were exposed to 1, 5, 10, and 20 ing/L ammonia-N for 9 weeks. Gonad specimens were taken from the fish and immediately weighed and fixed for histological sections and examined with light microscopy. The gonado-somatic index (GSI) decreased significantly (P < 0.05) with the increase of ammonia concentration. The percentage decrease of GSI was 28.94, 28.73, 49.18 and 55.28 % for 1, 5, 10 and 20 mg/L aminonia-N, respectively. The decrease of GSI was correlated with the increase of ammonia-N levels (r = -0.9193). The histological examination showed no remarkable changes in the oogenesis and spermatogenesis, at Img/L ammonia, but at higher concentrations of anmonia-N (5, 10 and 20 mg/L) there was a significant reduction (P<0.01) in the nunber of yolk vesicles and mature stages of the oocytes in the ovary and there was an inhibition in the spermatogenesis.