Six simple phenols in acid-hydrolyzed rat urine have been analyzed by gas chromatography. Following oral neomycin administration a reduction of 96 per cent was observed in the excretion of phenol, p-cresol, and 4-ethylphenol. Guaiacol, which is normally present in trace amounts, was not detected after neomycin treatment. The dihydric phenols, catechol, and 4-methylcatechol decreased by 88 per cent and 50 per cent respectively. After closure of the anal canal the excretion of phenol, 4-ethylphenol, catechol, and 4-methylcatechol decreased while the quantities of p-cresol in the urines increased. © 1969 Informa UK Ltd.