Cylindrical carbon fiber microelectrodes (CFMEs) (8 mm in diameter, 8 mm in length) are used to develop a voltammetric method for the determination of the thyreostatic drug methylthiouracil (MTU) in animal feed samples. Methanol, containing 0.05 mol L À1 tetrabutylammonium perchlorate as supporting electrolyte, was selected as working medium because of the high MTU solubility in this solvent, which is also used for the extraction of MTU from real samples. Reproducible electroanalytical responses were obtained with no need of applying cleaning treatments to the CFME. The net-current square-wave (SW) oxidation signal of MTU was selected to be exploited analytically. Under the optimized SW conditions, a linear calibration plot for MTU was obtained in the 1.0610 À6 -1.0610 À4 mol L À1 range, with a detection limit of 3.8610 À7 mol L À1 . The SW voltammetric behavior of other thyreostatic drugs was checked. A simple extraction procedure with methanol, followed by a clean up step, was used for the determination of MTU in a commercial dry fodder sample spiked with this drug at the 500 mg g À1 level. The detection limit in this type of sample was found to be 8.6610 À6 mol L À1 .