Articular chondrocytes experience changes to matrix hydration during both physiological (static load) and pathophysiological (osteoarthrosis, OA) conditions. Such changes should alter chondrocytes' volume, which has been shown to modify matrix metabolism. However, the osmometric behaviour of chondrocytes is not well understood. Here, using confocal laser scanning microscopy (CLSM), we have investigated the 'passive' osmotic responses of fluorescent-labelled chondrocytes within, and isolated from, the matrix. The volume-regulatory pathways normally activated by cell shrinkage/swelling, were blocked by bumetanide/REV5901, respectively. Chondrocytes in situ were broadly grouped into superficial (SZ), mid (MZ) and deep (DZ) zones, and there was a significant increase in resting cell volume with depth into the cartilage. Variation in medium osmolarity (range 0-530 mOsm; corresponding to an extracellular osmolarity of approximately 150 to approximately 600 mOsm) caused a rapid and sustained change to in situ MZ chondrocytes' volume. Over the range 180-380 mOsm, the change to in situ or isolated chondrocytes' volume was similar. For MZ chondrocytes. ideal osmometric (Boyle-van't Hoff) behaviour was apparent over the extracellular osmolarity range of approximately 250 to approximately 600 mOsm. Chondrocytes within the SZ appeared to be more sensitive to reduced osmolarity, swelling more for a given reduction in osmolarity, than MZ or DZ chondrocytes. These data show that over wide variations in osmolarity, articular chondrocytes in situ were osmotically sensitive, and for MZ chondrocytes behaved as perfect osmometers with the extracellular matrix (ECM) not restraining cell volume changes. Changes to matrix hydration may therefore alter passive chondrocytes' volume and unless compensated by volume-regulatory pathways, could lead to changes in cell volume, and hence matrix metabolism.