The involvement of naphthaleneacetic acid (NAA), a synthetic auxin, and methyl-2-chloro-9-hydroxy-fluorene-9-carboxylate (chlorflurenol), an auxin transport inhibitor, in fruit set in cucumber (Cucumis sativus L.) was investigated. In the greenhouse, exogenously applied NAA and chlorflurenol enhanced fruit set depending on the position of application. In the growth chamber, when NAA was applied directly on the stigma, a small quantity moved into the ovary after 24 hours. However, when auxin was applied on the ovary, significant quantities of 14C NAA were detected in the ovary and other plant pieces. When NAA was placed on the peduncle, more accumulated in peduncle and stem sections than when NAA was placed on the stigma or ovary. Chlorflurenol applications to the peduncle inhibited movement of NAA out of the ovary and resulted in accumulation of auxin in that tissue. NAA movement in the intact plant was similar to that of excised sections: chlorflurenol applied to the peduncle reduced NAA movement out of the ovary. Thus, the regulation of fruit set by chlorflurenol may be through the restriction of auxin movement from the ovary. The increased auxin accumulation in the ovary may trigger fruit set and subsequent growth.