Inclination of shoots of osier willow (Salix viminalis) from the vertical position results in outgrowth of buds on the upper side, whereas those on the lower side remain inactive. This effect was shown to be caused by gravity, and not by light or mechanical strain. The effect of gravity is not mediated through factors translocated from the roots or from the apical regions. The buds on the lower side are apparently not inhibited by those on the upper side of the shoot. Gravity-mediated lateral transport of auxin from the upper to the lower half of the stem is not a controlling factor, but such transport within each half of the stem may be important. The angle subtended by the bud itself to the direction of gravity is not an important factor. Bud-break is thought to be controlled by a mechanism acting between the bud and the adjacent tissues and which is affected by the orientation of the bud and the adjacent tissues relative to gravity.