Rheological characterization of cells using passive particle tracking techniques can yield substantial information regarding local cellular material properties. However, limited work has been done to establish the changes in material properties of mechanically-responsive cells that experience external stimuli. In this study, cardiac fibroblasts plated on either fibronectin or collagen were treated with cytochalasin, mechanically stretched, or both, and their trajectories and complex moduli were extracted. Results demonstrate that both solid and fluid components were altered by such treatments in a receptor-dependent manner, and that, interestingly, cells treated with cytochalasin were still capable of stiffening in response to mechanical stimuli despite gross stress fiber disruption. These results suggest that the material properties of cells are dependent on a variety of environmental cues and can provide insight into physiological and disease processes. (C) 2012 Elsevier Inc. All rights reserved.