Previous studies showed that the equatorial diffraction spacing of the collagen molecules in mineralized tissues decreases when the tissue is dried and that the spacing in totally dried tissue is about the same (1.1 nm) whether mineralized or not. Here we report that spacing decreases were observed in both mineralized and unmineralized turkey leg tendon after soaking in various sodium chloride solutions up to 4.0 M concentration. The effect was seen by X-ray diffraction as well as by neutron diffraction. No effect was seen in turkey leg tendon soaked in 3.0 M ethylene glycol solution. The spacing in unmineralized tissue decreased from 1.459 +/- 0.011 nm in 0.15 M saline to 1.403 +/- 0.025 nm in 1.5 M saline, a change of 0.056 +/- 0.03 nm or 3.84%. In mineralized turkey leg tendon the corresponding spacings were 1.387 +/- 0.012 and 1.321 +/- 0.019 nm, a change of 0.046 +/- 0.02 nm or 3.4%. No significant dimensional change was noted in the thickness even though the equatorial diffraction spacing decreased by 3.4%. Electron microscopy showed the collagen fibrils within the mineralized turkey leg tendon to be surrounded by highly mineralized material. Presumably the composition of the extrafibrillar material is different from the intrafibrillar and therefore the extrafibrillar material is a different kind of composite. If it is assumed that the extrafibrillar material does not change dimensions significantly, then the collagen molecules in the fibrils can be mobile within the dimensionally stable cage-like structure.