1. The thromboxane-mimetic, U46619, was a more potent contractile agonist than prostaglandin D2 (PGD2), PGF2 alpha or histamine in human isolated bronchial and pulmonary arterial muscle preparations. 2. Human isolated proximal bronchial muscles were less sensitive to contractile agents than distal bronchial preparations. However, the former tissues developed a greater contractile force (Emax) when compared with results obtained in the latter tissues. 3. BAY u3405 attenuated the contractions induced by U46619, PGF2 alpha and PGD2 in both human isolated bronchial and pulmonary arterial muscle preparations. 4. BAY u3405 did not alter histamine concentration-effect curves nor the relaxation induced by Butaprost (TR4979) in human isolated bronchial muscle preparations. In addition BAY u3405 did not modify the relaxation induced by PGI2 in isolated pulmonary arterial muscle preparations. 5. The contractions induced by different prostaglandins were blocked by BAY u3405, suggesting a common functional site for these agents found both on human bronchial and pulmonary arterial muscles.