When pea plants (Pisum sativum L. cv Feltham First) are subjected to freezing conditions (-18 degrees C) followed by a thaw to 18 degrees C, there is a significant inhibition of water-splitting capacity judged by the rate of light-induced reduction of 2,6-dichlorophenol indophenol using isolated thylakoid membrane fragments enriched in photosystem II (PSII). The freeze-thaw-induced inhibition of water-splitting activity has been correlated with the loss of the 17- and 23-kilodalton extrinsic protein of PSII and with a weakening of the binding of the 33-kilodalton protein. There was no apparent loss of bound manganese. Addition of 10 millimolar CaCl(2), however, allowed a full recovery of the water-splitting activity of these modified PSII-enriched particles. The freeze-thaw-induced changes in the organization and functional capacity of PSII was found to increase its susceptibility to photoinhibition in agreement with the concepts presented in the accompanying paper, that oxidative damage can occur within the PSII reaction center as a consequence of extending the lifetime of P680(+).