The preparation and characterization of nitric oxide (NO)-releasing dendrimer conjugates are reported. Generation 3 and 5 polypropylenimine dendrimers (DAB-Am-16 and DAB-Am-64) were modified at the exterior to impart different amine functionalities. The ability to store NO on a dendritic scaffold using N-diazeniumdiolate NO donors was examined via the reaction of primary amine, secondary amine, and amide functionalities with high pressures of NO (5 atm). The secondary amine dendrimer conjugates exhibited a high storage capacity for NO (up to 5.6 mu mol NO/mg), greatly increasing the "payload" of released NO over existing macromolecular NO donors. The mechanism of diazeniumdiolate decomposition was proton initiated, generating NO spontaneously under physiological conditions (pH 7.4, 37 degrees C). The NO release durations (> 16 h) observed for the secondary amine dendrimers were significantly longer compared to small molecule alkyl secondary amine diazeniumdiolates, thus illustrating a dendritic effect on NO release kinetics. The multivalent exterior of dendrimers allows for the future combination of NO donors and other functionalities on a single molecular scaffold, enabling diverse utility as NO storage/delivery systems.