The bronchodilator drugs isoprenaline, salbutamol, theophylline and prostaglandin E1 (PGE1) relaxed the guinea-pig isolated tracheal chain preparation dose-dependently and their potencies were compared by EC50 values. 2. The non-steroid anti-inflammatory drugs flufenamate, mefenamate and phenylbutazone also relaxed the preparation dose-dependently, but were less potent than the sympathomimetic drugs and PGE1. 3. The contractile response to a submaximal concentration of prostaglandin F2alpha (PGF2alpha) was antagonized by flufenamate, mefenamate, phenylbutazone, aspirin, theophylline, isoprenaline, salbutamol and PGE1, at concentrations similar to or less than the clinical peak plasma levels in man. A relatively higher concentration of indomethacin was required. 4. The antagonism was relatively specific for PGF2alpha in the case of the fenamates, which did not cause comparable reductions in responses to equi-effective concentrations of histamine and carbachol. The other anti-inflammatory drugs, theophylline and the sympathomimetic drugs were less or non-specific. 5. The nature of the shifts in the long dose-response curve for PGF2alpha caused by increasing concentration levels of flufenamate indicates a dual competitive/non-competitive type of antagonism.
