In Chenopodium rubrum seedlings (ecotypes 50°10′ N and 49°58′ N) betacyanin synthesis is light dependent (completely dark-grown seedlings contain no betacyanin) and is under phytochrome control via both the low energy and the high-energy (HER) reactions of photomorphogenesis. In continuous light, accumulation of betacyanin is linear with time. However, when a single dark period interrupts continuous light, the amount of both betacyanin and chlorophyll synthesized during a given period of time after the dark interruption shows a rhythm reflecting differences in the rate of, and (or) the capacity for, pigment accumulation that are dependent on the duration of the dark period. The rhythm in chlorophyll content was higher in frequency than circadian, with a period of about 15 h, while rhythmicity in the rate of synthesis of betacyanin was circadian. These results suggest that there is endogenous rhythmicity in the metabolic state of the system in darkness. The imposition of light after darkness apparently stabilizes the specific physiological status attained at that respective time of darkness and thus determines the metabolic activity of the seedlings.When glucose was supplied throughout darkness interrupting continuous light, the phasing of the rhythm of betacyanin synthesis was positively correlated with the rhythm of flower initiation, but this was not so when phenylalanine was supplied during darkness. In contrast, when glucose was supplied for a varied length of time in continuous light, there was rhythmicity in the rate of betacyanin accumulation, with a periodicity of about 15 h, that was dependent on the duration of the glucose application.When seedlings were supplied with 10−6 M gibberellic acid during darkness there was a rhythm in the amount of hypocotyl elongation that depended on the length of a single dark period interrupting continuous light. Other evidence has suggested that there is a rhythm in the stability of the cellular membranes; this rhythm was assayed (non-physiologically) by the time of onset of betacyanin leakage from seedlings into an extraction medium and was apparent only after application of 10−10 M gibberellic acid. The rhythms in hypocotyl elongation and in membrane stability that were revealed after the application of gibberellic acid suggest that there may be a rhythm in the rate of differentiation and (or) development of the system.It is postulated that endogenous rhythmicity is due to the spatial separation of energy production and use in different cell particulates, with phytochrome acting as a membrane operator.