Hormonal response in Cyprinus carpio to sub-lethal Dimethoate toxicity was investigated by estimating the serum levels of T3 (triiodothyronine), T4 (thyroxine), cortisol, prolactin, and insulin in control and sub-lethal (0.50 mg/l) dose for the periods of 1h, 6h, 12h, 24h, and 3 days. In control Cyprinus carpio, the serum level of T3 ranged from 0.81 ± 0.01 to 0.85 ± 0.02 ng/ml; T4 from 1.95 ± 0.01 to 1.99 ± 0.01 ng/ml; cortisol from 12.21 ± 0.01 to 12.26 ± 0.01 µg/dl; prolactin from 0.54 ± 0.01 to 0.59 ± 0.01 ng/dl; and insulin from 1.69 ± 0.01 to 1.75 ± 0.01 µU/ml up to maximum period of 3 days maintained in a pollution-free tap water. Exposure to the sub-lethal level of dimethoate cause stress in the fish as is indicated by a change in the serum hormone levels. From the results, it can be concluded that Dimethoate causes a decrease in metabolic rate and indirectly reduces the toxic effect of pesticide as is evident from the reduced levels of T3 and cortisol. Increased level of insulin is known to favor adaptive tissue glycogenesis, besides its role in increasing the lipogenesis that may be advantageous for the fish in sequester pesticide residues, thereby increasing pesticide tolerance in the fish. Increased prolactin level indicates a possible hydromineral regulatory effect on the kidney and gills of fish under pesticide toxicity. The fish appears to adaptively recover its hormonal profile following a prolonged exposure period of 24 h to 3 days.