The imidazolinylhydrazones of (3-pyridinyloxy)-acetaldehyde and of 6-[3-(2-formyl-pyridinyl)oxy]hexanoic acid were synthesized as cyclic analogues of the corresponding guanylhydrazones which were found to be selective inhibitors of human thromboxane-synthase. The benzene isosters were also prepared in order to define the importance of the ring nitrogen for the activity. Moreover, the guanyl- and imidazolinyl-hydrazones of two 6-[(3-pyridinyl)oxy]hexanoic acids showing in the 2 position an alkyl chain with an alpha, beta-unsaturated ketonic function were prepared. Imidazolinylhydrazones 7 and 18 are selective inhibitors of thromboxane-synthase, while the two guanylhydrazones 14 and 15 which do not affect prostanoid biosynthesis seemed to be antagonists at the thromboxane receptor.