The concentration of several phenolic acids and alcohols was measured in urine from germ-free and specific pathogen-free (SPF) rats before and after inoculation with faecal microorganisms, and from conventional rats before and after gut sterilization. The rate of excretion of benzoic acid, phenylacetic acid, and m- and p-hydroxyphenylpropionic acid in the germ-free animals was markedly increased after inoculation. Some acids showed no increase, including the endogenously generated homovanillic, vanilmandelic and p-hydroxyphenyllactic acids. Most others sought showed a small but significant increase. Some of the compounds excreted by the germ-free animals may have been in the food pellets, either as such or as precursors. The pattern was somewhat different in the SPF rats. The excretion of p-hydroxyphenylpropionic, p-hydroxyphenylacetic and m-hydroxyphenylacetic acids was initially much higher than in the germ-free animals and their excretion decreased after inoculation, presumably because of an altered pattern of gut flora. This work quantifies the effect of gut flora in the formation of some of the more important phenolic acids found in rat urine.