Apparatus is described in which the root system of plants can be exposed to accurately known external stresses in beds of ballotini for periods up to 3 weeks. A constant flow of aerated nutrient solution was supplied. Modifications of the apparatus allowed the emerging coleoptile also to be subjected to mechanical stress. Earlier observations that roots are unable to decrease in diameter to enter small pores were confirmed. The effect of external pressure on the rate of root elongation was independent of the diameter of pores provided this was smaller than the penetrating root. Pressures of 20 and 50 kPa reduced the elongation of the seminal axes of barley by 50 and 80 per cent respectively. Significant reductions in root extension were also caused by 50 kPa or less in more limited experiments with a number of other crop plants. The extension of the coleoptile and the first leaf responded similarly to that of the roots. When the diameter of pores in the rooting medium was intermediate between those of axes and laterals, the former being impeded, an enhanced development of laterals led to a dense superficial root system. This considerable modification of root form was without effect on nutrient uptake provided that an adequate supply of nutrients was available in the restricted rooting zone. Attention is directed to some of the implications of these responses for plant growth under field conditions. However, consideration of the physiological mechanisms responsible for the effects of low external pressures and the changes in cell structure to which they lead are deferred to subsequent papers.