To ensure masculinization and normal growth of the obligately air-breathing fighting fish Betta splendens, norethindrone acetate (NE) was administered through discrete immersions for 3 h each on the second, fifth and eighth day post-hatching (dph) at selected concentrations (250, 500, 750, 1000 and 1250 μg/l). Immersions at 1000 μg/l ensured the maximum of 92% masculinization, 71% survival, normal growth and appearance of secondary sexual characteristics. NE reduced the air-breathing frequency on the fifth and eigth dph. It also reduced the frequency in males depurated for 172 days following the treatment, although males treated at 750 μg/l almost restored the frequency. In the ovary of the treated but persisting females, it reduced the number of vitellogenic oocytes and increased the vacuolar area. In the testis of the treated males, it reduced the number of spermatogonia and increased the vacuolar area. The treated males could neither induce the female to spawn as many eggs nor accommodate as many fertilized eggs within the bubble nest, as that of the control. While the control females attained puberty on the 140th dph and spawned 122 eggs once every 16 days, the persisting females, which were previously treated at 1000 μg/l, postponed puberty to the 183rd dph and prolonged the inter-spawning period to 40 days. During the 240-day experiment, NE reduced the cumulative progeny production from 764 to 104 (13.6% of the control)
