Xyloglucan endotransglycosylase (XET) activity extractable from internodes of tall and dwarf varieties of pea (Pisum sativum L.) was assayed radiochemically using tamarind seed xyloglucan as donor substrate and an oligosaccharidyl-[3H]alditol as acceptor substrate. Internodes I and II showed little elongation during the period 15 to 21 d after sowing; XET activity remained relatively constant and was unaffected by exogenous gibberellic acid (GA3). A single application of GA3 to the dwarf genotype resulted in a small enhancement of elongation in internode III between d 17 and 21 and caused a small increase in XET activity in internode III. Repeated applications of GA3 caused internode V to elongate between d 20 and 26, to the same extent as in the tall variety, and concomitantly led to greatly elevated XET activity (expressed per unit fresh weight, per unit of extractable protein, and per internode). Thus, XET activity correlated with GA3-enhanced length in pea internodes; the possibility that this represents a causal relationship is discussed.