In a short-day response type of Chenopodium rubrum (ecotype 60°47′ N), light of a relatively low red/far-red ratio—but of sufficient energy to allow photosynthesis—can bring about induction of flowering when it completely replaces a single dark period interrupting continuous white light. When high-intensity white incandescent light was interrupted for less than a 24-hour period, a longer period of inductive light than darkness was required even for minimal induction. An inductive light interruption of at least 60 hours was required for 100% flower induction. The result of such forcing of the system by inductive light, as compared with the circadian rhythmic induction that occurred in darkness, was a change towards a more linear inductive response and there were indications (requiring confirmation) of oscillations of higher frequency.When seedlings were maintained continuously in optimal inductive light or in darkness, after an initial high intensity white light period, there was some flower initiation within 5 days in inductive light, but not until about 10 days in darkness, and then only when sucrose was supplied throughout darkness.There were suboptimal and (inhibitory) supraoptimal effects on induction when the R/FR ratio and (or) the energy of inductive light were decreased or increased, respectively. These results, in conjunction with the effects that were obtained when glucose and 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) were applied in inductive light as compared with darkness, implicate both photosynthate and phytochrome-Pfr as having a positive (promotive) effect during normal inductive darkness.These findings emphasize that the important controls in photoperiodism and flowering may be quantitative rather than qualitative in character, because it can now be questioned whether there is any essential dark-requiring reaction in the induction not only of long-day but also of short-day plants.