Pea epicotyls (Pisum sativum, cv. Alaska) were enclosed in chambers in which their elongation was restricted by means of a foam neoprene stopper or by a medium of glass beads. These treatments increased evolution of ethylene and resulted in reduced length and increased diameter of both the internodes and the cells of the internodes. These responses increased with increasing degrees of restriction. A time-sequence study of the emergence of epicotyls through 90 mm of glass beads showed that an accelerated evolution of ethylene preceded a reduction in elongation. As the epicotyls elongated through the glass bead medium and less resistance was encountered, evolution of ethylene declined and rapid elongation was resumed. The morphological and anatomical effects of a 120-mm column of glass beads were duplicated by applied ethylene concentrations of 0.2 ppm or less. Evolution of CO2 was inhibited slightly by the ethylene treatments. The data indicate that production of ethylene by pea epicotyls is increased by nonwounding physical stress, and that the ethylene acts as an endogenous growth regulator, decreasing elongation and increasing diameter in response to increasing increments of stress.