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1. Hypotonic solutions added to human platelet-containing plasma cause a transient decrease of absorbancy of light at 610 mµ which is followed by a gradual increase of absorbancy.

2. When platelets are stored for 7 hr at 4° C the absorbancy changes with variations of osmolarity and their aggregation with adenosine diphosphate (ADP) remain the same. However, the reversal of absorbancy declines during storage of platelet-containing plasma.

3. Platelets are not aggregated by stearate. Platelets appear to be only slightly affected by stearate concentration higher than 0·8 mM, but oleate has no effect.

4. Hypertonic solutions of NaCl and urea cause increase in absorbancy of platelet-containing human plasma. Hypertonic sucrose solutions produce no more change than isotonic solutions. Hypertonic NaCl produces permanent increases in absorbancy. In human platelet-containing plasma the increased absorbancy caused by hypertonic urea is transient and declines.

5. The osmotic platelet changes occur in isolated platelets as well as in platelet-containing plasma.

6. The absorbancy of frozen and thawed platelet-containing plasma is not significantly altered by hypotonic solutions but the absorbancy changes caused by hypertonic solutions are similar to that of unfrozen plasma.

7. The immediate absorbancy changes caused by hypo- and by hypertonic solutions are the same at 5° C and 30° C and are therefore probably of a physical nature. The reversal of absorbancy and aggregation of platelets by added adenosine diphosphate have Q₁₀ > 1 and are therefore probably of a chemical-enzymic nature.

8. Divalent cations and contact activation are not required for the osmotic platelet changes and 10^-3 M-Cu²⁺ and Zn²⁺ do not interfere. Inhibitors of oxidative phosphorylation, electron transfer, sodium, potassium activated adenosine triphosphatases and adenosine triphosphate do not inhibit reversal of absorbancy of platelets exposed to hypotonic solutions. Cyanide, 5 x 10^-3 M, fluoride, 1·23 x 10^-2 M, iodoacetamide, 10^-2 M, are moderately effective inhibitors. At hydrogen ion concentrations above pH 8, complete inhibition occurs.

9. N-ethyl-maleimide, 10^-3 M, and mercloran inhibit completely reversal of absorbancy, indicating the necessity for sulphydryl compounds.

10. Intact platelets are essential for the reversal of absorbancy after hypotonic swelling. Osmotic changes by hypotonic solutions are independent of ADP aggregation of platelets.