Earlier work in 1985 and 1986 indicated that cotton petiole was a site of significant nitrate reduction. Those studies used a modified in vivo assay of nitrate reductase activity (NRA). The plant materials were incubated under air with 5% propanol in the inbubation medium. The study in 1987 employed a conventional in vivo assay with 1% propanol in the incubation medium. The plant materials were vaccum infiltrated and then shaked under nitrogen gas for 1 h. Results of field samples confirmed our earlier work. The 1987 study also indicated that when plants were at four mature leaf (vegetative) stage petiole and blade had similar NRA. As plants progressed into reproductive stages, petiole had higher NRA, e.g. at squaring stage, petiole had a rate of 0.92 µmol NO2/g FW/h and blade had 0.27. At the squaring stage, the contribution of petiole to shoot NRA was 24%, which was significant considering its contribution of 12% to shoot fresh weight. At the same stage, the contribution of blade to shoot was 582 and 33% for fresh weight and NRA, respectively. In a comparison between petiole and blade at the early squaring stage over ten cotton varieties, the petiole contribution to the top mature leaf NRA was varied between 28 to 68%, much higher than its contribution of 19% to the leaf fresh weight. We conclude that cotton petiole in vivo NRA was significant and may be used as an index of for the recommendation of N fertilization.