Curly dock (Rumex crispus L.) seeds failed to germinate in the dark, but germinated readily in light at 15, 20, and 25C. A secondary dormancy was induced in these seeds by 6 days of dark or light incubation at 30C (thermodormancy), by 6 days of dark incubation at 15C, or by a prolonged soaking in −15.7 bar polyethylene glycol-6000 (PEG) solution at 15C in light. The secondary dormancy induced at 15C was partially removed by 2-chloroethylphosphonic acid (ethephon) in presence of light; however, when the dormancy was induced at 30C, an addition of gibberellin A4+7 (GA4+7) to ethephon was necessary for effective removal of the dormancy. The ability of growth regulators to remove the secondary dormancy induced by prolonged dark incubation at 15C decreased progressively in light as the duration of dark incubation increased. The water-potential studies indicate that the thermodormancy can be attributed, at least in part, to the inability of the embryonic cells to lower their water potential to a level sufficient to cause radicle protrusion.