The goal of this study was to determine whether injections of serotonin increase the plasma concentration of thyroxine (T4) , triiodothyronine (T3), growth hormone (GH), milk amount and constituents in the cows. sixty cows were randomly divide into 4 groups. Each group received daily injections of either saline or 1, 2, 4 milligrams serotonin per Kg for 15 days. Blood samples were collected daily on day 3 before injections untill 3 days after the last injection . Samples were assayed for plasma T3, T4 and GH concentrations of the cows by double-antibody RIA. Milk samples were daily collected from day 1 day till day 21 of experiment. Milk samples were assayed for protein, fat and lactose constituents. The daily amount of milk and BW changes were determined throughout the experiment. Injections of saline, 1 and 2 milligrams serotonin per Kilograms did not changed the plasma concentration of the T3, T4 and GH during the period of injections in the all animals. Injection of 4 milligrams serotonin per kilograms significantly increased the plasma concentration of the T3, T4 and GH during injections in the animals. The increase of GH plasma concentrations was shown since day 7 and T4 and T3 plasma concentrations was increased since day 11 through out the experiment. Injections of saline, 1 and 2 milligrams serotonin did not changed the amount of milk in the animals. Injections of 4 milligrams serotonin significantly increased the amount of milk by 20%. The protein, fat and lactose constituents of milk were not changed among all animals in the different groups. The result of these experiment indicated that long injection of serotonin may increase the plamsa concentration of GH, T3, T4 and the amount of milk in the cow.
Key word : serotonin, T3, T4, GH, cow