The anti-inflammatory, analgesic and antipyretic effects and the tolerability of 2-(4-isobutylphenyl)-propionic acid (ibuprofen) were compared with those of its derivative 2-(4-isobutylphenyl)-propiohydroxamic acid (ibuproxam, Ibudros. Our experiments show that the interfering action of the two drugs with the reactive processes of the tibio-tarsic articulation, brought about by carrageenin, serotonin, dextrane and formalin is of the same intensity. Also, the analgesic activity is perfectly consistent with the antipyretic one. However, the tolerability of the two molecules is different; the acute toxicity is consistent in the case of the single parenteral administration and it differs in the case of a single or repeated oral treatment. When used under these conditions, ibuproxam is considerably less damaging to the gastroenteric tube than is ibuprofen. The hypothesis is put forth that the greater tolerability of ibuproxam is due to its pharmacokinetics: it is, as such, little or not toxic for the mucous membrane of the digestive apparatus, and it progressively releases ibuprofen, whose concentrations in the blood would remain below those levels that cause systemic lesions in the gastroenteric tract.