The existence of a diurnal fluctuation in root metabolism can be deduced from several studies. While studying root respiration, Harris (13) noted a diurnal variation in the rate of CO2 production by roots of tomatoes and certain deciduous fruit trees. Cannon (5) found oxygen consumption of roots to be increased in light under conditions conducive to high transpiration. Grossenbacher (7), Speidel (24), and Went (26) found autonomic diurnal fluctuations in root pressure and exudation from decapitated plants; Hagan (8) observed basically the same phenomenon in non-exuding root systems, and showed it dependent on aerobic respiration. Arisz, Helder, and van Nie (2), van Andel (1), and Vaadia (25) have extended these observations to implicate the transfer of salt to the xylem elements as the process primarily concerned. Hanson and Biddulph (11) demonstrated a diurnal variation in ion transfer to the shoot of intact bean plants grown at constant temperature and humidity under artificial light, which Hylmo (16) believes may be associated with increased transpiration during the period of illumination. Went (26) observed that the accumulation of p32 in the shoots of tomato plants was more rapid during the day. These diurnal fluctuations in root activity could be a reflection of a diurnal variation in respiration rate, as suggested by the experiments of Harris (13). However, Grossenbacher (7) found no correlation between fluctuations in root pressure of excised root systems and respiration. Since the completion of our work Vaadia (25) has published experiments showing no diurnal variation in the respiration or ion uptake of excised roots or root sections. On the other hand, one of us (Hageman) had observed a diurnal variation in root respiration of intact Derris plants; these results are reported here. The objective of this work was to determine if there was in fact a diurnal variation in root respiration. Respiration measurements were accordingly made with excised root tissue and the roots of intact corn and soybean plants. The experiments showed intact roots to respire more rapidly during daylight hours, but root tissue excised from these plants exhibited no such diurnal variation.