Background and purpose: The pharmacological properties of compounds NCX 1512 and NCX 1514, synthesized by linking the histamine H1-receptor antagonist cetirizine to NO-releasing spacer groups, are reported. The aim was to establish if the compounds retained the antihistamine action of the parent compound, to assess their efficacy as NO donors and to test if they had broader antiallergic activity than cetirizine in the lung. Experimental approach: Antihistamine activity of NCX 1512 and NCX 1514 was investigated in vitro in the guinea pig ileum, in tracheal rings (GPTR) and lung parenchymal strips (GPLP) of the guinea-pig. The NO-releasing capacity was investigated in vascular preparations; the isolated rabbit and guinea-pig aorta and guinea-pig pulmonary artery. Kinetics of NO release were assessed in a rat whole blood assay. Key results: Both NCX 1512 and NCX 1514 retained activity as H1-receptor antagonists in the guinea pig ileum and airway preparations. The NO-releasing NCX compounds relaxed the rabbit aorta, an action prevented by the guanylyl cyclase inhibitor ODQ (10 mM). NCX 1512 and NCX 1514 did not relax the antigen (ovalbumin) pre-contracted GPTR, whereas the NO donors NCX 2057 and DEA-NONOate relaxed guinea-pig pre-contracted vascular and tracheal preparations. Cetirizine (1-100 mM) and NCX 1512 (1-100 mM) reduced the cumulative (0.01-100 mg ml À1 ) ovalbumin-induced constriction in GPTR, but had no significant effect in GPLP. Conclusions and implications: NCX 1512 and NCX 1514 act as antihistamines and NO donors. However, there was no improved effect compared to cetirizine on antigen-induced constriction of the central and peripheral lung. (2007) British Journal of Pharmacology