The elongation growth of 5-mm tip sections of 3-day-old etiolated wheat coleoptiles was promoted by light flashes of 436, 650 and 750 nm in comparison with the dark control. As short as 7/1,000 sec red light (R) of 14,420 W/m2 led to saturation of the R-induced growth response. At quantum-identical far-red light (FR) irradiation, a plateau developed at about 9/1,000 sec which, however, was interrupted at longer irradiation periods. Varying dark periods between R or FR activation of growth and a second FR flash, a light-independent period with a half-life of 45 to 60 sec was found. Only after this light-independent period could R or FR activation of growth be reversed by FR. A second light-independent period was found by the following irradiation schedule: 1 sec R/100 sec darknees(D)/l sec FR/varied D/l sec R. Our experimental conditions enabled us (a) to distinguish between activation and inactivation by FR, (b) to clarify the reaction chain of different phytochrome forms and (c) to determine the half-life of light-independent phytochrome reactions.