The visible and near-uv absorption and circular dichroic spectra were determined for spinach and poplar plastocyanin under a variety of conditions. The visible spectra showed that the copper center was invariant to changes in species, chemical modification with ethylenediamine, and addition of high concentrations of salt [2.7 M (NH4)2SO4]. In contrast, the near-uv spectra were sensitive to these conditions. Reduction of plastocyanin also altered its near-uv absorption and circular dichroic spectra. It is unlikely that these spectral changes were due to charge transfer bands since the near-uv CD spectrum of apo-plastocyanin was almost identical to that of reduced plastocyanin. There were no corresponding changes in the far-uv spectra which monitor protein secondary structure. The most likely explanation is that the protein has a flexible tertiary conformation. Conformational changes may be important in regulating electron transport. If plastocyanin is a mobile electron carrier, differential binding of the oxidized and reduced forms of plastocyanin to its reaction partners cytochrome f and P700 could facilitate electron transport.