The pyridine antihistamines carbinoxamine maleate and doxylamine succinate were derivatized by hydrolysis with cyanogen bromide and coupling with aniline to form a polymethine dye, and determined by colorimetry. In a micellar cationic medium ofN-cetylpyridinium chloride (NCPC), the reaction times were reduced and the sensitivities improved. In 0.1 M NCPC the apparent molar absorptivities of the derivatives were 5,900 and 3,500 M−1cm−1, and the limits of detection (as 3s) were 8.6x10−7M and 1.7x106M, for carbinoxamine and doxylamine, respectively. The sensitivity was enhanced in a three- to six-fold factor with respect to a non-micellar medium. The reproducibility was 0.8% for 1.6x10−4M carbinoxamine, and 1.8% for 1.3x10−4M doxylamine. The procedure was applied to the evaluation of the drugs in pharmaceutical preparations.