The effects of in vivo preillumination with short wavelength red light (647 nm) and long wavelength red light (710 nm) on fluorescence and photochemical characteristics of algal and higher plant chloroplasts were analyzed and data are correlated to previous studies of divalent ion effects on isolated chloroplasts. The 647 nm light increased the sensitization of photosystem I as determined from fluorescence emission at low temperature, decreased room temperature variable yield fluorescence, decreased the sigmoidal aspect of fluorescence rise, and increased the rate of photosystem I reactions measured in low intensity 647 nm light. DCMU blocked 647 preillumination effects. These data are consistent with a previous suggestion that preillumination effects on chloroplast fluorescence are related to light-induced ionic redistributions in the organelle. Structural organization of pea chloroplasts preilluminated with either 647 or 710 nm light were not significantly different with respect to the extent of grana stacking observed. The differences between membrane structural reorganization observed after preillumination and those observed with chloroplasts in the presence or absence of salts is discussed.