The physiological roles of CO in neurotransmission, vasorelaxation, and cytoprotective activities have raised interest in the design and syntheses of CO-releasing materials (CORMs) that could be employed to modulate such biological pathways. Three iron-based CORMs, namely, [(PaPy3)Fe(CO)](ClO4) (1), [(SBPy3)Fe(CO)](BF4)(2) (2), and [(Tpmen)Fe(CO)](ClO4)(2) (3), derived from designed poly-pyridyl ligands have been synthesized and characterized by spectroscopy and X-ray crystallography. In these three Fe(U) carbonyls, the CO is trans to a carboxamido-N (in 1), an imine-N (in 2), and a tertiary amine-N (in 3), respectively. This structural feature has been correlated to the strength of the Fe-CO bond. The CO-releasing properties of all three carbonyls have been studied in various solvents under different experimental conditions. Rapid release of CO is observed with 2 and 3 upon dissolution in both aqueous and nonaqueous media in the presence and absence of dioxygen. With 1, CO release is observed only under aerobic conditions, and the final product is an oxo-bridged diiron species while with 2 and 3, the solvent bound [(L)Fe(CO)](2+) (where L = SBPy3 or Tpmen) results upon loss of CO under both aerobic and anaerobic conditions. The apparent rates of CO loss by these CORMs are comparable to other CORMs such as [Ru(glycine)(CO)(3)Cl] reported recently. Facile delivery of CO to reduced myoglobin has been observed with both 2 and 3. In tissue bath experiments, 2 and 3 exhibit rapid vasorelaxation of mouse aorta muscle rings. Although the relaxation effect is not inhibited by the soluble guanylate cyclase inhibitor ODQ, significant inhibition is observed with the BKCa channel blocker iberiotoxin.