The effect of far-red light on the growth of the second internode was studied in Helianthus annuus L. The plants were grown in a greenhouse during the early stages of development, then transferred to a growth chamber where they were exposed to 12 h light and 12 h darkness. Far-red treatments were initiated when the second internode was 1 mm long and were continued until the second internode attained its final length, a period of 2-3 wk. The experimental plants were subjected daily to far-red irradiation at the beginning of the dark period, for 4-32 min in dose-response studies, and for 16 min in all other studies. The internodes of plants that were exposed to supplementary far-red light grew at a faster rate and became longer than those of the control plants. There was no significant difference in the duration of the growing period. Under the conditions of these experiments, the longer the exposure to far-red irradiation, up to 16 min, the greater the promotion of growth; but there was no further increase when the irradiation was extended to 32 min. Histological studies on the pith showed that the longer internodes of the treated plants were composed of longer cells and of a larger number of cells than those of the control plants. Irradiation of selected parts of the shoot system demonstrated that the increase in growth of the internode was the result of far-red light acting on both the internode and on other parts of the shoot system.