Growth substances were measured in xylem exudate from the stem stumps of detopped maize (Zea mays L. cv. Inra 200), whose roots had been grown in culture solution at either 8, 13, 18, 23, 28, or 33 °C, between 30 and 47 d after sowing. After extraction of the exudate and gel chromatography, bioassay was used to detect three cytokinin, four gibberellin, and four growth-inhibitor fractions. It was found that the export of total cytokinin and gibberellin was greatest at a root temperature of 28 °C, while inhibitor export was lowest at 33 °C, the root temperature that was optimal for shoot extension growth. As the root temperature was lowered to 8 °C, shoot growth became progressively more restricted, but while there was a concomitant decline in cytokinin and gibberellin export, there was an increase in the export of growth inhibitors. It is suggested that poor maize shoot growth during prolonged low root-temperature treatments, such as at 8 and 13 °C, may be related to an altered balance between the growth promoters and inhibitors that are exported to the shoot from the roots.