Time measurement during the light period in the photoperiodic induction of flowering in Xanthium pennsylvanicum has been studied, primarily by one type of experiment: a subcritical dark period (7.5 hours) is terminated by a brief but saturating interruption of light; and varlous conditions (light periods) are then given to discover treatments which will result in optimum flowering in response to a second inductive dark period (typically 12 hours). 1. Experiments with applied sucrose, as well as the general tenor of most of the experiments in this paper, indicate that the High Intensity Light Process is more than photosynthesis. Sucrose, for example, will not overcome the inhibitory effect of a light interruption during a dark period. 2. Following the first subcritical dark period (optimally 7.0 to 7.5 hours), a light period of 8 to 12 hours will result in maximum flowering in response to a second, test-dark period (12 hours). There is little if any response to light intensity during this intermediate light period. This is true for low intensities on the order of those required for saturation of the phytochrome pigment system (intensities comparable to those effective in the inhibition of the dark period) to intensities which might saturate photosynthesis. 3. Red light is most promotive of flowering during the intervening light period, and far-red light is inhibitory. Thus the light process is apparently a phytochrome response, opposite in direction to the Pfr inhibition of the dark period. 4. Time measurement during the light period is apparently quite temperature intensitive from 15° to 38°C. 5. As the light period is lengthened, the critical dark period is shorted. With a 12-hour light period, the critical dark period has been shortened to less than 8 hours. As the light period is lengthened beyond 12 hours, the critical dark period increases towards its usual “lower” limit of 8.3 hours. 6. If time measurement is studied by determination of the time during the dark period when the flowering process is maximally sensitive to a red light interruption, it can be seen that the process is considerably slowed during dark periods which follow a very short light interruption. After a 12-hour light interruption, the time of maximum sensitivity is reached even earlier (ca. 6 hours) than under “normal” conditions (ca. 8 hours). 7. Results are discussed according to the concepts of catenary steps in the flowering process or rhythmical changes in the plant's sensitivity to light. It is concluded that both approaches have merit, and that some blending of the two ideas probably comes closest to description of the true state of affairs. The plant seems to clearly go through a diurnal cycle, during half of which red light (Pfr) is promotive, even essential to flowering, and during the other half of which red light (Pfr) is inhibitory. Yet catenary steps in the process are also discernible, such as a conversion of the phytochrome pigment system by light or metabolically in the dark, coupling through this system to the timing mechanism, synthesis and translocation of flowering hormone after a sufficient period of time, and synthesis of an inhibitor (apparently an inhibitor of time measurement during the critical dark period) during light periods longer than about 12 hours.