By means of histochemical procedures the characterization of the compartments of articular cartilage, the distribution of water, electrolytes, connective tissue, and chondroitin sulphage in these compartments during immobilization atrophy have been described. The results obtained support the following conclusions concerning the cartilage during immobilization of an extremity:

1. The total water content increased as the result of an increase in the ultrafiltrate volume in the extracellular compartment.

2. The amounts of both the connective tissue and the chondroitin sulphate in fresh cartilage decreased but these percentage decreases were not so large as when 100 grams of cartilage solids were considered, suggesting that the low values of these two constituents were the result of dilution of the extracellular compartment with the increased ultrafiltrate volume.

3. The ?Na values (total determined sodium minus the sodium in the extracellular phase) were low paralleling the low chondroitin sulphate values and thus giving further support to the conclusion that the low chondroitin sulphate values were the result of dilution.

4. In the intracellular compartment of the chondrocytes, the percentage of intracellular water did not change; nor was there any change in the concentration of potassium which is the cation present in the chondrocyte indicating that the functioning cytoplasm of the chondrocytes had not altered during immobilization atrophy.

5. Considering the two types of experimental atrophy in cartilage from denervation reported previously and disuse reported here, our chemical data and their interpretations have afforded a basis for some deductions on when atrophy is reversible, as in disuse, and when it is not reversible, as in atrophy following denervation.