Amaranthus tricolor half-seedlings show a greater accumulation of betacyanin in response to a defined period of red light if given a pretreatment at an elevated temperature (40 C). Red light given before the shift to 40 C is ineffective. The maximum response is achieved after a 2-hour shift to 40 C and if the red light is given 1 hour after return to the germination temperature (25 C). The effect on red light induction of betacyanin synthesis and on fusicoccin induction by these conditions is similar, whereas the increase in cytokinin-dependent synthesis is greater. Both phytochrome-cytokinin synergism and fusicoccin-cytokinin synergism are changed in the same manner by this treatment. Phytochrome and fusicoccin responses are inhibited similarly by carbonyl-cyanide-p-trifluoromethoxy-phenylhydrazone.If the seedlings are again returned to 40 C after the 25 C shift there is an inhibition of betacyanin synthesis. The interpretation of these results is that a shift to 40 C induces the formation of some component necessary for the full expression of phytochrome potential, that for the maximum formation or action of this component it is necessary for the seedlings to spend 1 to 2 hours at a lower temperature (for example 25 C) before red light is given and that for the subsequent processes in betacyanin synthesis an elevated temperature is inhibitory.