The role of various growth-regulating substances in tuberization in Solarium andigena was studied (1) by application of exogenous growth regulators, and (2) by studying variation in the levels of endogenous hormones under tuber-inducing and non-inducing conditions. In short-day induced cuttings tuberization was inhibited by gibberellic acid; indole acetic acid and kinetin (6-furfuryl amino purine) caused slight stimulation of tuber growth at low concentrations, but inhibited it at higher concentrations. Application of (2-chloroethyl) trimethyl-ammonium chloride (CCC) promoted tuberization in plants growing under non-inducing long-day conditions and in buds produced on isolated plugs of tuber tissue. The tuber-inducing effects of CCC were not reversed by simultaneous application of GA3. Levels of endogenous gibberellins decreased markedly under short-day conditions, but there was no difference in the levels of endogenous abscisic acid under long and short days. The reduced levels of gibberellins were maintained for at least 2 weeks when induced plants were transferred from short-day to long-day conditions. Application of CCC under conditions which promoted tuberization caused a reduction in gibberellin levels. The role of growth hormones in the regulation of tuberization is discussed.