The primary thickening of the stem segment of pea (Pisum sativum L. var. Alaska) has been studied from the viewpoint of growth substance and histology. It has been found that the thickening is induced by the simultaneous administration of IAA and kinetin, but not by either of them given separately, and that the thickening is due to an increase in cell size of the epidermis, cortex and pith, unaccompanied by an increase in cell number. It is discussed that the thickening of the intact stem is regulated by kinetin or a kinetin-like substance(s) in the plant in co-operation with endogenous auxin.