In vitro data support a scheme of phytochrome phototransformation involving intermediates in a sequential pathway. The fraction of total phytochrome maintained as intermediate under conditions of pigment cycling as well as the rate of the dark reversion of the far red-absorbing (Pfr) to the red-absorbing form of phytochrome (Pr) has been shown to depend on the molecular environment of the phytochrome molecules. Inverse dark reversion of Pr to Pfr has been observed in vitro. These results contribute toward an understanding of the observed paradoxes between physiological experiments and measurements of the amount and state of phytochrome in vivo. The in vivo spectrophotometric assay measures an average of the properties of phytochrome in different cellular environments, whereas a particular physiological response may be controlled by phytochrome molecules in one particular environment. It is therefore possible that all phytochrome is potentially active and triggers specific responses by virtue of its localization.