Low concentration fumigation with nitric oxide (NO*) has been shown to extend the postharvest life of a range of flowers, fruits and vegetables by down-regulating ethylene production. Since ethylene is involved in flower abscission and leaf senescence of `John Fanick' phlox cut flower heads, a superior selection of perennial phlox (Phlox paniculata L.) bearing attractive long-lasting flowers, we have evaluated the effect of NO* delivered in vivo using sodium nitroprusside (SNP) as the source of NO* donor, on postharvest performance of `John Fanick' phlox flower heads. Although the presence of SNP (10-200 μmol·L-1) in the vase solution promoted the abscission of the open flowers, the young flower buds continued to open even in the presence of high SNP concentrations. On the other hand, at high SNP concentrations, the leaves became either yellow, or more frequently turned progressively black and senesced. Inclusion of sucrose in the vase solution, or pretreatment of flower heads with either 1-MCP or STS, significantly delayed the abscission of flowers and blackening of leaves. The pretreatment of flower heads with either 1-MCP or STS, or the presence of sucrose in the vase, together with SNP, greatly reduced the toxicity of the latter chemical resulting in improved postharvest display life. These results indicate that in `John Fanick' the leaves are relatively more susceptible to NO*-induced toxicity than the flowers. However, both sucrose and ethylene perception inhibitors are able to minimize the toxicity of high concentrations of NO* delivered in vivo via SNP.