IMPROVED WELL DRILLING FLUIDS CONTAINING A SMALL BUT EFFECTIVE AMOUNT OF A THINNING AGENT SELECTED FROM THE GROUP CONSISTING OF CERTAIN SULFONATED POLYHYDRIC PHENOLS AND CERTAIN SULFONATED POLYHYDRIC NAPHTHOLS CAPABLE OF BEING OXIDIZED TO A QUINONE, CERTAIN METAL SALTS OF SAID PHENOLS AND SAID NAPHTHOLS, AND OXIDATION PRODUCTS OBTAINED UPON OXIDATION OF SAID PHENOLS, SAID NAPHTHOLS, AND SAID METAL SALTS OF SAID PHENOLS AND SAID NAPHTHOLS; AND METHODS OF DRILLING A WELL USING SAID DRILLING FLUIDS.