Field pennycress (Thlaspi arvense L.) is a winter annual weed with a cold requirement for stem elongation and flowering. The relative abilities of several native gibberellins (GAs) and GA-precursors to elicit stem growth were compared. Of the eight compounds tested, gibberellin A(1), (GA(1)), GA(9), and GA(20) caused stem growth in noninduced (no cold treatment) plants. No stem growth was observed in plants treated with ent-kaurene, ent-kaurenol, ent-kaurenoic acid, GA(53), or GA(8). Moreover, of the biologically active compounds, GA(9) was the most active followed closely by GA(1). In thermoinduced plants (4-week cold treatment at 6 degrees C) that were continuously treated with 2-chlorocholine chloride to reduce endogenous GA production, GA(9) was the most biologically active compound. However, the three kaurenoid GA precursors also promoted stem growth in thermoinduced plants, and were almost as active as GA(20). No such increase in activity was observed for either GA([unk]) or GA(53). The results are discussed in relation to thermoinductive regulation of GA metabolism and its significance to the initiation of stem growth in field pennycress. It is proposed that thermoinduction results in increased conversion of ent-kaurenoic acid to GAs through the C-13 desoxy pathway and that GA(9) is the endogenous mediator of thermoinduced stem growth in field pennycress.